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Lesson 2 Matter

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Course: Science 10 [5 cr] - AB Ed copy 1
Book: Lesson 2 Matter
Printed by: Guest user
Date: Sunday, 7 September 2025, 6:43 PM

Table of contents

Introduction

Did you know that chemistry is the study of matter?



B2.1 Chemistry classroom
It is easier to study matter if we categorize it and look at the properties and characteristics of each of those categories. Once we have learned how to categorize matter, we will spend the majority of this lesson focused on atomic theory.

  Target

By the end of this lesson, you will be able to

  • identify the laboratory evidence that scientists such as Dalton, Thomson, Rutherford, and Bohr collected during the development of the atomic model, which consists of protons, neutrons (nucleons), and electrons.

  Watch This

The 2,400-year search for the atom – Theresa Doud @ youtube TED-Ed 


This video will give you a quick summary of the historical milestones in the development of atomic theory. This will help get you in the right mindset for this lesson.

Classifying Matter

Did you know that according to the Chemical Abstracts Registry Service, there are more than 90 million unique chemical substances?



B2.2 Various chemical formulas
B2.3 Element
Knowing that, it may seem like studying chemistry is a huge, impossible task! But once we categorize matter based on similar properties, this task becomes manageable.

Matter is anything that takes up space and has mass. Matter is categorized as either a pure substance or a mixture. Pure substances contain only one material, while mixtures are combinations of substances that can be separated by physical means.

Pure substances can be further classified as elements and compounds. Elements are substances that only contain one type of particle and cannot be broken down further by chemical means. Compounds are combinations of two or more elements and these can be broken down into simpler substances (elements) by chemical means.
B2.4 Compound

B2.5 Solution
Mixtures can be subdivided into two categories: homogenous and heterogeneous. Homogenous (solutions) mixtures look as if they contain only one material—they look uniform. Heterogeneous mixtures are composed of different components that are distinctly visible.

B2.6 Mixture

The rest of this lesson will focus on elements and the fundamental particle that elements are composed of—the atom.

  Read This

Please read pages 12 to 15 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on classifying matter. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. Classify each of the following substances as an element, compound, homogenous mixture, or heterogeneous mixture.

    Substance Classification
    soil
    limestone (CaCO3)
    apple juice
    orange juice with pulp
    gold
    baking soda (NaHCO3)
    propane (C3H8)
    tap water
    the Bow River
    oxygen
    air
    vinegar
    iron filings
    table sugar
    rusty iron
    mercury
    milk
    chromium
    acetylene (C2H2)
    paper

    Substance Classification
    soil heterogeneous
    limestone (CaCO3) compound
    apple juice
    		homogeneous
    orange juice with pulp heterogeneous
    gold element
    baking soda (NaHCO3) compound
    propane (C3H8) compound
    tap water
    		homogeneous
    the Bow River
    		heterogeneous
    oxygen element
    air homogeneous
    vinegar homogeneous
    iron filings
    		element
    table sugar
    		compound
    rusty iron
    		heterogeneous
    mercury element
    milk heterogeneous
    chromium element
    acetylene (C2H2) compound
    paper heterogeneous

Evolution of Atomic Matter

Don’t trust atoms
they make up everything!



B2.7 Evolution of atomic theory
Early scientists or philosophers such as Democritus, Aristotle, and Socrates developed theories based on hypothesizing instead of experimenting. With the development of the scientific method, evidence-based theories became the standard for science. The following pages highlight some of the key historical developments in atomic theory. Each scientist built upon the knowledge of the previous model, and through collaboration, scientists have modified historical theories into a unifying atomic theory (known as the quantum mechanical model of the atom). But scientists are not done; even this model is still being modified, as better equipment, mathematical models, and scientific understanding arise.

  Dalton's Atomic Theory

“It’s the right idea, but not the right time.” –John Dalton



© Wikimedia Commons
B2.8 John Dalton

An analogy that can be used to help you visualize Dalton’s model of the atom is a solid sphere like a billiard ball.
© Wikimedia Commons
 B2.9: Dalton’s atom
Around 1804, English chemist John Dalton resurrected Democritus’s idea of the atom. While investigating properties of gases and observing how chemicals reacted and recombined, he inferred that matter must contain tiny, individual particles that are in constant, random motion.

In addition to scientific observation, Dalton also used two known laws to help develop his theory: the law of conservation of mass and the law of constant composition.

The five main points of Dalton’s atomic theory are as follows:

  1. All matter is made of atoms that are indivisible and indestructible.

  2. All atoms of a given element are identical in mass and properties.

  3. Atoms of different elements have different masses and properties.

  4. Compounds are formed by a combination of two or more different kinds of atoms and always in the same ratio.

  5. Atoms are not destroyed during a chemical reaction; rather they are rearranged.

His theory became widely accepted because it could be used to explain many previously unexplainable observations.

Even though his theory is over 200 years old and some of his theory has been disproved, many of his points are still valid.

  Did You Know?

© Wikimedia Commons
B2.10 Dalton’s symbols for different elements

Dalton was the first to use standard symbols for elements. These symbols are quite different from the element symbols on the modern periodic table!

  Read This

Please read page 22 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on John Dalton’s atomic theory. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. The contribution John Dalton made to atomic theory was his discovery that

    1. every atom was positively charged
    2. every element consisted of one type of atom
    3. atoms had nuclei
    4. atoms could be divided into smaller parts

  2. True or False: Dalton thought that atoms were made up of smaller particles.

    False. Dalton thought that atoms were the smallest particles that could not be subdivided farther.


  3. Write the five points of Dalton’s atomic theory in your own words. This means that you cannot copy the points down; you must use words that make sense to you.

  Thomson's Atomic Theory

Did you know that older-style television sets use cathode ray tubes to create the image?



© Wikimedia Commons
B2.11 J. J. Thomson
The next key development in the atomic theory occurred in the late 1890s. British physicist J. J. Thomson was investigating cathode rays. Cathode rays are created in a cathode ray tube, which is a glass tube with almost all of the air removed. The tube also contains two pieces of metal, one at each end. When an electric current is applied to one of the pieces of metal, a ray or beam can be seen traveling through the tube. Initially, scientists did not understand what these rays were. To try to gain a better understanding of cathode rays, Thomson tested them by placing negative and positive plates along the sides of the cathode ray tube. The cathode ray was repelled by the negative plate and attracted by the positive plate. This indicated that the ray was composed of negatively charged particles. (Recall the law of charges; Opposite charges attract and like charges repel.) Thomson repeated his experiments using different metals and found that the properties of the cathode ray remained constant no matter what cathode material they originated from. He concluded that these subatomic particles must be found within atoms of all elements and that they are negatively charged.

© Wikimedia Commons
 B2.12 Cathode ray tube

  Did You Know?


B2.14 Scale

Thomson also measured the mass of the particles he identified. He did this by determining how much the cathode rays bent when he varied the voltage. He found that the mass of the particles was 2,000 times smaller than the mass of the smallest atom—the hydrogen atom.

This disproved Dalton’s theory that atoms are the smallest particles of matter. Thomson stated that the structure of an atom contained randomly distributed negative particles that he called corpuscles (later renamed electrons). Since he knew that atoms were neutral, he theorized that the remainder of the atom was a positively charged sphere. He called his atom “the plum pudding model.” In this analogy, the pudding part is the positive part of the atom and the embedded raisins are negatively charged electrons.
© Wikimedia Commons
 B2.13 Thomson model of the atom
© Wikimedia Commons
 B2.14 plum pudding

  Virtual Lab

Cathode Ray Tube © The Concord Consortium


Please review this simulation to help you visualize the experimental evidence that Thomson collected that helped him develop his plum pudding model of the atom.

Click on the procedure tab to continue.

  1. Click on the play icon to open the virtual lab.The lab can also be found at https://quick.adlc.ca/cathode
  2. Change “Adjust charge on horizontal plates” to “None.”
  3. Leave electrode material as “silver.”
  4. Check the “TURN ON” box to start the simulation.
  5. Select “Display beam.”
  6. Observe.
  7. Select “Display particles.”
  8. Change “Adjust the charge on horizontal plates” to “Very high +/–.”
  9. Which way do the particles move?

    Down, toward the positive plate
  10. What does this indicate about the charge on the particles?

    Must be negative if it is attracted to a positive charge.
  11. Select “Display beam.”
  12. Change “Adjust the charge on horizontal plates” to “Very high –/+.”
  13. Which way does the beam bend?

    Up, toward the positive plate
  14. What does this indicate about the charge of the beam?

    Must be negative if it is attracted to a positive charge.


  Read This

Please read pages 22 and 23 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on J. J. Thomson’s experiment and his atomic model. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. Describe the evidence that J. J. Thomson collected in the cathode ray tube experiment.

    Particles flew out of metal when a current was applied. When charged plates were brought close to this beam or ray of particles, they deflected toward the positive plate.
  2. Describe how Thomson interpreted the evidence he collected in the cathode ray tube experiment.

    There were two key interpretations.

    1. Smaller particles than atoms existed.

    2. The beam of particles that were emitted through the cathode ray tube bent toward the positive plate, and since opposite charges attract, the beam must consist of negative particles.
  3. Describe how Thomson modified Dalton’s model of the atom in light of the evidence gathered during the cathode ray tube experiment.

    The atom could no longer be a solid sphere; it had to contain subatomic particles that were negatively charged. To maintain the neutrality of the atom, the remainder of the atom must be positively charged.
  4. How would you describe J. J. Thomson’s model of the atom? What analogy would you use for J. J. Thomson’s model of the atom? Use your own words!

  Rutherford's Atomic Theory

One of the key aspects of the scientific method is to try to disprove a theory. Only if a theory can withstand rigorous testing is it deemed to be valid.


© Wikimedia Commons
 B2.15 Ernest Rutherford
In 1911, physicist Ernest Rutherford performed an experiment to test Thomson’s plum pudding model. In the experiment, Rutherford shot very small alpha particles (which are positively charged helium ions) at a thin sheet of gold foil. Rutherford expected all of the particles to be deflected just a bit as they passed through the “plum pudding.” He found that most of the alpha particles he shot at the foil were not deflected at all. They passed through the foil and emerged undisturbed. Occasionally, however, particles were scattered at huge angles and a few of them even bounced right back.

  Did You Know?

B2.20 Geiger counter

Rutherford worked with Professor Hans Geiger in creating the Geiger counter. A Geiger counter is a devise that is used to detect radiation, which can have serious health effects. Click on the photo to listen to the detection of radiation.

  Watch This

Rutherford's Alpha Scattering Experiment ©Blausen


Watch this video that will provide an overview of Rutherford’s experiment and how he interpreted the results.

  Rutherford's Atomic Theory Continued


© Wikimedia Commons
B2.16 Rutherford’s gold foil experiment
To understand how surprising these results were, imagine shooting a rifle at a mound of loose snow. You would expect most bullets to emerge from the opposite side. But imagine your surprise if a bullet reflected back at you! If that happened, you might guess that there was a brick of hard material inside the snow mound.

These results lead Rutherford to propose that most of an atom was empty space but with a positively charged center that contained most it's mass—Rutherford had discovered protons and the nucleus (from the Latin for “little nut”). Rutherford also proposed that the nucleus contained a neutral particle, which was eventually named “the neutron.” But it was not until 1932, that James Chadwick was able to prove that these neutral particles exist.
© Wikimedia Commons
B2.17 Expected pathway of alpha particles in the Thomson model and the Rutherford model of the atom

  Virtual Lab

Rutherford Experiment © The Concord Consortium


Work through this simulation to help visualize how changing the placement of the positive charges will affect the deflection of alpha particles.

Click on the procedures tab to continue.

  1. Click on the play icon to open the virtual lab. The lab can also be found at https://quick.adlc.ca/rutherford
  2. Uncheck “Show field generated by positive charges.”
  3. Move the cursor for “Set Spread of Positive Charge” to maximum diffuse.
  4. Click “Shoot alpha particles.”
  5. Observe the pathways and interactions of the alpha particles with the positive charges.
  6. Run the simulation for 20 s. Then click “Stop.”
  7. Take a screen shot of the results.
  8. Click “Reset.”
  9. Move the cursor for “Set Spread of Positive Charge” to maximum concentrated.
  10. Click “Shoot alpha particles.”
  11. Observe the pathways and interactions of the alpha particles with the positive charges.
  12. Run the simulation for 20 s. Then click “Stop.”
  13. Take a screen shot of the results.
  14. Click on the analysis tab to complete the analysis questions.
  1. What pattern, similar to the Thomson model of the atom, did the alpha particles make when the spread of positive charge was set to diffuse?

    Most were slightly deflected.


    © Concord Consortium
    B2.18 electron pathway through the atom


  2. What pattern, similar to the Rutherford model of the atom, did the alpha particles make when the spread of positive charge was set to concentrated?

    Some were deflected at great angles.


    ©Concord Consortium
    B2.19 electron pathway showing electrons colliding with the nucleus


Many years later, reflecting on his reaction to these results, Rutherford said, "It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."


  Rutherford's Atomic Theory Continued


By recording the number of the alpha particles deflected at large angles, Rutherford was able to estimate the size of the nucleus. According to his calculations, the radius of the nucleus is at least 10,000 times smaller than the radius of the atom!

© Wikimedia Commons
B2.21 Model of Rutherford’s atom
An analogy that can be used to help you visualize Rutherford’s model of the atom is a planetary model (sometimes called a nuclear model), which puts all of the protons in the nucleus and the electrons orbiting around the nucleus.

  Digging Deeper

© Wikimedia Commons
B2.22 nuclear powerplant

The idea that the nucleus of an atom is made up of smaller particles also led to the first splitting of an atom (nuclear fission). Two students of Rutherford, Ernest Walton and J.D. Cockcroft, successfully split a lithium ion in 1932. This was a crucial first step in the development of nuclear power.

  Watch This

Rutherford’s Experiment: Nuclear Atom @ YouTube HerrPingui 


Watch this video to review and visualize Rutherford’s gold foil experiment that provided evidence for the existence of a nucleus and protons.


  Read This

Please read page 23 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on Ernest Rutherford’s experiment and his atomic model. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. Describe the evidence that Rutherford collected in the gold foil experiment.

    He shot alpha particles at thin gold foil to watch for any pathway changes of the alpha particles as they passed through the gold foil. He was expecting most to pass through with slight deflection. In reality, most passed through without being deflected, with a few being deflected substantially.
  2. Describe how Rutherford interpreted the evidence he collected in the gold foil experiment.

    There were two key interpretations.

    1. Since most alpha particles were deflected, atoms are mostly empty space.

    2. Since some alpha particles were deflected at huge angles, the interior of the atom must contain a central core that is very dense and positively charged.
  3. Describe how Rutherford modified Thomson’s model of the atom in light of the evidence gathered during the gold foil experiment.

    There had to be a central core that contained almost all of the mass and the positive charges, instead of it being spread evenly about the atom. The atom had to be composed of mostly empty space, with electrons circling around it.
  4. How would you describe Rutherford’s model of the atom? What analogy would you use for Rutherford’s model of the atom? Use your own words!

  Bohr's Atomic Theory

Have you ever bought colour crystals for your fire pit and wondered where the colours came from?


The different colours come from different elements. If you sprinkle table salt on a fire, you get a yellow colour from the sodium. Salts that contain potassium give a purplish flame. If you look at the flames through a spectroscope (an instrument that uses a prism to break up light into its various components), you will see a number of lines of various colours. Those distinct lines of colour make up that element’s emission spectrum, which you may recall learning about in Science 9.

Niels Bohr, a Danish scientist, explained emission spectrum while developing a model for the atom.
© Wikimedia Commons
B2.23 Fireworks

© Wikimedia Commons
B2.24 Niels Bohr with Albert Einstein
In 1913, Bohr came to work in the laboratory of Ernst Rutherford. Rutherford asked Bohr to work on improving his model of the atom. The Rutherford model did well to explain the results of the gold foil experiment; however, it had a major drawback: It could not explain why electrons do not fall into the nucleus when taking a spiral path. In the Rutherford model, electrons revolved in circular orbits of varying sizes. According to scientific understanding at the time, any charged particle moving on a curved path emits energy and thus the electrons would lose energy and spiral into the nucleus. This meant Rutherford's planetary atom should have an extremely short lifetime.

Bohr thought about the problem at the same time he was studying the emission spectrum of hydrogen. He quickly realized that the two problems were connected, and after some thought, he came up with the Bohr model of the atom. Bohr's model of the atom revolutionized atomic physics.

Bohr modified the Rutherford model by requiring that the electrons move in orbits of fixed sizes and energies.

© Wikimedia Commons
B2.26 Emission spectrum of hydrogen
B2.25 electron spiraling into the nucleus

The Bohr model was based on his observations of the atomic emission spectrum of the hydrogen atom. When white light is diffracted with a prism, all the colours of the visible spectrum can be seen. Each colour corresponds to a specific amount of energy. When the light given off by the hydrogen atom was viewed through a spectroscope, only certain colours of light could be seen. This led Bohr to theorize that electrons only have certain energies in an atom and they had to be in energy levels. Variations in the amount of energy are seen as the light of different colours.

  Did You Know?

B2.29 Bohrium from the periodic table

The chemical element bohrium (Bh), No. 107 on the periodic table of elements, is named for Niels Bohr.
Bohr received the Nobel Prize in 1922 in physics for atomic structure and quantum mechanics. He was one of the most influential scientists of the 20th century.

  Digging Deeper

The energy level an electron normally occupies is called its ground state. But it can move to a higher-energy, less-stable level, or shell, by absorbing energy. This higher-energy, less-stable state is called the electron’s excited state.

After it’s done being excited, the electron can return to its original ground state by releasing the energy it has absorbed.

Sometimes the energy released by electrons occupies the portion of the electromagnetic spectrum (the range of wavelengths of energy) that humans detect as visible light. Slight variations in the amount of the energy are seen as light of different colours.

B2.30 electron jumping energy levels

B2.27 Electrons circling in set energy levels
In 1913, Bohr proposed his shell model of the atom. To make Rutherford’s model more stable, Bohr gave a new arrangement of electrons in the atom. Electrons could revolve around the nucleus only in certain energy levels, each having a different radius. The energy of an electron depends on the size of the energy level and is lower for smaller orbits. The Bohr model shows that the electrons in atoms are in orbits of differing energy around the nucleus (think of planets orbiting around the sun).

Main Points of the Bohr Model:

  • Electrons orbit the nucleus in energy levels that have a set size and energy.
  • The energy of the level is related to its size. The lowest energy is found in the smallest level, which is closest to the nucleus.
  • Energy is absorbed or emitted when an electron moves from one level to another.

Bohr also proposed the following ideas about atomic structure:

  • The number of electrons in the outer orbit determines the properties of an element.
  • Energy levels can hold differing numbers of electrons: Energy level 1 may hold up to two electrons, energy level 2 may hold up to eight electrons, and so on.

© Wikimedia Commons
B2.31 electron orbiting the nucleus
Bohr’s model was also called a planetary model. The difference between Rutherford’s and Bohr’s models is that Bohr had the electrons orbiting at specific distances from the nucleus.

The Bohr model of the atom was quickly disproved, because only hydrogen behaved as expected. However, at the high school level, modified Bohr diagrams are still useful to help students start to visualize and understand the underlying structure of atoms.

  Read This

Please read page 24 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on Neils Bohr’s experiment and his atomic model. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. What was the problem with Rutherford’s model that Bohr was trying to remedy?

    The electrons should lose energy and spiral into the nucleus of Rutherford’s model.
  2. In Bohr’s atomic model, __________ travel in definite paths around the ________ at specific levels. Each level is a certain __________ from the nucleus.

    In Bohr’s atomic model, electrons travel in definite paths around the nucleus at specific levels. Each level is a certain distance from the nucleus.
  3. What evidence did Bohr use to support his model?

    Bohr looked at the emission spectrum for hydrogen.
  4. Why was Bohr’s model rejected?

  Quantum Mechanical Model of the Atom

Just as crawling is the first developmental step to mobility, earlier models of the atom lead the way to the modern quantum mechanical model of the atom.



B2.32 Baby crawling
Bohr knew his model needed to be modified. It needed to be able to predict/explain elements other than just hydrogen. Over the next several years, many scientists, including Louis de Broglie, Erwin Schrödinger, and Werner Heisenberg, collaborated and contributed to the development of the quantum mechanical model of the atom.

Recall that in the Bohr model, the exact path of the electron was restricted to circular energy levels around the nucleus. In actuality, electrons do not travel around the nucleus in simple circular orbits. The quantum mechanical model gives the probability of finding an electron at a given point around the nucleus. This model can be portrayed as a positive nucleus surrounded by an electron cloud.

Where the cloud is densest, the probability of finding the electron is greatest and the electron is less likely to be in a less dense area of the cloud. This model is based on probability rather than certainty.

The quantum mechanical theory is very complex and includes complex mathematical equations, the understanding of quantum theory (which says that matter has properties associated with waves), and the application of the uncertainty principle.
© Wikimedia Commons
B2.33 Atom model at the American Museum of Science and Energy in Oak Ridge


  Try This

Modeling an Electron Cloud


The electron cloud can be modeled in the following way.

  1. Place a piece of paper on the floor with a red dot in the center, representing the nucleus.
  2. Standing over the paper, take a black marker with your arm extended straight out and try to drop it onto the red dot.
  3. You should observe small marks at each point the marker hits.
  4. Repeat many, many times.
  5. Click on the analysis tab to complete the analysis questions.
  1. What does each dot represent?

    Each dot represents a location where the electron could be at any given moment. Because of the uncertainty principle, there is no way to know exactly where the electron is.
  2. What does the overall pattern look like?

    The overall pattern of dots will be roughly circular.
  3. Are the dots evenly distributed?

    There will be more dots near the nucleus and progressively fewer dots as you move away from it. An electron cloud has variable densities: a high density where the electron is most likely to be and a low density where the electron is least likely to be.

  Interactive Activity

Structure of an Atom © The Concord Consortium


This simulation shows the possible location of electrons at various moments in time. You can explore the pattern of where electrons will most likely be located

Click the procedure tab to continue.

  1. Click on the play icon to open the interactive activity. The interactive can also be found at https://quick.adlc.ca/alpha
  2. Slide “Delay between finding Electrons” all the way to “long.”
  3. What do you notice about the pathway of the electrons?

    The motion is random and variable in distance from the nucleus.
  4. Check “Trace Electrons.”
  5. Click “Start.”
  6. Let simulation run for 25 s.
  7. Click “Stop”
  8. What do you notice about the distribution of the electrons?

    They are more concentrated around the nucleus.
  9. Uncheck “Trace Electrons.”
  10. Slide “Delay between finding Electrons” all the way to “short.”
  11. Click “Start.”
  12. How many electrons does it look like there are? Explain.

    It appears as many more than two electrons. This is because the electrons are moving so fast.
  13. Click “Stop.”
  14. How many electrons are there?

  15.  Check “Trace Electrons.”
  16. Click “Start.”
  17. Let simulation run for 25 s.
  18. Click “Stop.”
  19. What do you notice about the distribution of the electrons?

    They are more concentrated around the nucleus.

  Quantum Mechanical Model of the Atom Continued


The quantum mechanical theory still incorporates the concept of energy levels; the pathways are just not as restricting as Bohr’s model. It goes further to also incorporate energy sub-levels, orbitals, and electron spin.

Although Rutherford suspected there was a third subatomic particle, this was not proven until 1932. In 1932, James Chadwick bombarded beryllium atoms with high energy particles. This caused another particle—with a neutral electrical charge and the approximate mass of a proton—to come loose. This particle became known as the neutron.


B2.34 Model of an atom


Since 1932, through continued experimentation, many additional particles have been discovered in the atom, such as quarks that make up protons and neutrons. Also, new elements have been created by bombarding existing nuclei with various subatomic particles. The study of the composition of the atom continues to be an ongoing and exciting journey.

  Read This

Please read pages 25 and 32 to 33 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on the quantum mechanical model of the atom. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. What particles are found in the nucleus of the atom? Describe these particles.

    The proton and the neutron. The proton is a positively charged particle, while the neutron is a neutral particle. Both the proton and neutron are of approximately the same size.
  2. What is the third particle found in the atom? Where is it located? How does it compare to the particles found in the nucleus?

  Periodic Table and Atomic Structure

All atoms have the same basic structure, but how do atoms of different elements differ?


B2.35 Stylized model of an atom
An atom is made up of three particles: electrons, protons, and neutrons. Electrons have a negative charge, protons have a positive charge, and neutrons have no charge—they are neutral. Due to the presence of an equal number of negative electrons and positive protons, the atom as a whole is electrically neutral.

The protons and neutrons (collectively called nucleons) are located in a small nucleus at the centre of the atom. Due to the presence of protons, the nucleus is positively charged.

  Watch This

What Is an Atom? @ YouTube MonkeySee 


This video is a good summary of the structure of the atom.




  Periodic Table and Atomic Structure

Now that you understand the basic structure of an atom, let’s look at how different elements are structured. Throughout this course, we will use a modified Bohr model to help visualize the structure of the atoms of the first 20 elements.

Here is the structure of the modified Bohr model.
B2.36 Modified Bohr model of an atom

A special name is given to the last energy level that contains electrons; it is called the valence energy level. This concept will be very important later on when we talk about the bonding of atoms to form compounds.

In order to model an element using the modified Bohr diagram, there are three key pieces of information that are needed: the number of protons, the number of electrons, and the number of neutrons. This information is readily available from the periodic table found in your data booklet.  If you do not have a copy of the Science 10 data booklet, please contact your teacher.

Key information about the composition of atoms is listed on the periodic table.
B2.37 Legend of the periodic table

Some of the information is self-explanatory, such as element name and symbol. The following clarifies the other terms and information.

The atomic number is the number of protons in an atom. Each element has a different number of protons.

Examples

Click on the video beside each example to see a teacher work through the example.
Example 1: An atom is found to have 35 protons in its nucleus. What element is this atom? https://adlc.wistia.com/medias/5opgb5dlc9

 

 



Bromine

 

 

 


Recall that atoms are neutral particles, meaning they contain the same number of protons and electrons. So in a neutral atom, the atomic number will also represent the number of electrons.

Atomic molar mass is the average mass number of all naturally occurring isotopes, based on percent abundancy.

Isotopes are different forms of the same element. They differ by having a different number of neutrons. We will discuss this concept in more detail later on.

Mass number, which is not on the periodic table, is the number of protons plus the number of neutrons in an isotope of an element.

Using mass number and atomic number, you can calculate the number of neutrons in an isotope.

The symbol of an isotope is slightly different, as it specifies the mass number and the number of protons for that specific isotope.

«math» «mstyle mathsize=š24pxš» «mmultiscripts» «mo»§#160;«/mo» «mprescripts»«/mprescripts» «mrow» «mi»atomic«/mi» «mo»§#160;«/mo» «mi»number«/mi» «/mrow» «mrow» «mi»mass«/mi» «mo»§#160;«/mo» «mi»number«/mi» «/mrow» «/mmultiscripts» «mi»element«/mi» «mo»§#160;«/mo» «mi»symbol«/mi» «/mstyle» «/math»

Symbols for the two stable naturally occurring isotopes of nitrogen are

«math»«mstyle mathsize=š24pxš»«mmultiscripts»«mo»§#160;«/mo»«mprescripts»«/mprescripts»«mn»7«/mn»«mn»14«/mn»«/mmultiscripts»«mi mathvariant=šnormalš»N«/mi»«mo»§#160;«/mo»«mo»§#160;«/mo»«mo»§#160;«/mo»«mo»§#160;«/mo»«mo»§#160;«/mo»«mmultiscripts»«mo»§#160;«/mo»«mprescripts»«/mprescripts»«mn»7«/mn»«mn»15«/mn»«/mmultiscripts»«mi mathvariant=šnormalš»N«/mi»«mo»§#160;«/mo»«/mstyle»«/math»

The element name of a specific isotope is also slightly different, as it includes the mass number of that isotope.

For example, nitrogen consists of two stable isotopes that are named nitrogen-14 and nitrogen-15.

  Digging Deeper

Some isotopes are extremely important, such as the isotope carbon-14, which is used for carbon dating. Watch this video.

B2.42 Fossils

This website identifies all of the isotopes for each element on an interactive periodic table.

Learn More

You may wonder why the atomic molar mass is listed as 14.01 on the periodic table when the two isotopes have mass numbers of 14 and 15; shouldn’t the average be 14.5? But remember that it is based on percent abundancy. Of all the naturally occurring nitrogen, approximately 99.6% is nitrogen-14 whereas approximately only 0.4% is nitrogen-15.

Examples

Click on the video beside each example to see a teacher work through the example.

Example 1: Identify the number of protons, electrons, and neutrons in an atom of potassium-41. https://adlc.wistia.com/medias/qfgb8iio1m ;
 

B2.40 potassium from the periodic table
The atomic number represents the number of protons. An atom is neutral, so it will have the same number of electrons.

Protons = 19 and electrons = 19

Determine the number of neutrons by subtracting the number of protons (atomic number) from the mass number.

Number of Neutrons = Mass Number – Atomic Number

Number of neutrons = 41 – 19

Number of neutrons = 22

Example 2: Identify the number of protons, electrons, and neutrons in an atom of copper-65.  https://adlc.wistia.com/medias/7syc003bpb
 

B2.41 copper from the periodic table
The atomic number represents the number of protons. An atom is neutral, so it will have the same number of electrons.

Protons = 29 and electrons = 29

Determine the number of neutrons by subtracting the number of protons (atomic number) from the mass number.

Number of Neutrons = Mass Number – Atomic Number

Number of neutrons = 65 – 29

Number of neutrons = 36
On the periodic table, a general name (no mass number) and symbol (no mass number or atomic number) are shown, and this, not the isotope notation, is what will be used most often in high school chemistry.

The ion charge and ion name are very important and will be discussed more in Section 2 of this unit.

  Constructing a Modified Bohr Model

Let’s look at an example to build your skills on reading information from the periodic table and using it to construct a modified Bohr model. Click on the video to watch a teacher work through this example.

Draw a modified Bohr diagram for an atom of phosphorus-31.
 


On the periodic table, look up the atomic number of phosphorus. This is the number of protons and will be placed in the nucleus.

B2.43 phosphorus from the periodic table


Determine the number of neutrons by subtracting the number of protons (atomic number) from the mass number.
Number of Neutrons = Mass Number – Atomic Number
Number of neutrons = 31 – 15
Number of neutrons = 16
Place the number of neutrons in the nucleus.
Determine the number of electrons. Recall that atoms are neutral, so the number of electrons equals the number of protons. For this atom, it will have 15 electrons.

Draw in the first energy level and place two electrons at the top. Recall that the first energy level can hold a maximum of two electrons.
There are still 13 more electrons to place. Place one electron in each orbital of the second energy level. If there are more than four electrons, start to pair up electrons to a maximum of eight electrons.
There are still five more electrons to place. Place one electron in each orbital of the third energy level. Since there are more than four electrons, pair up electrons on the top orbital.


  Read This

Please read pages 33 to 34 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on information about the structure of elements that are located on the periodic table. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. Which subatomic particle determines the identity of an element?

    The protons determine the identity. Each element has a unique number of protons. This relates to the atomic number on the periodic table. For example, if you know that a certain element has 16 protons, that element must be sulfur.
  2. How many electrons and protons are in an atom of chlorine?

    There are 17 protons and 17 electrons. This number is on the periodic table; it is the atomic number for chlorine.
  3. Identify the number of protons, electrons, and neutrons in an atom of magnesium-26.

    protons = 12, electrons = 12, and neutrons = 14. To determine the number of protons, look up the atomic number for magnesium on the periodic table. For this isotope, the mass number is 26, so to determine the number of neutrons, subtract the atomic number from the mass number: 26 – 12 = 14
  4. Write the isotope symbol for calcium-44.

    «math xmlns=šhttp://www.w3.org/1998/Math/MathMLš» «mstyle mathsize=š24pxš» «mmultiscripts» «mo»§#160;«/mo» «mprescripts»«/mprescripts» «mrow» «mi»atomic«/mi» «mo»§#160;«/mo» «mi»number«/mi» «/mrow» «mrow» «mi»mass«/mi» «mo»§#160;«/mo» «mi»number«/mi» «/mrow» «/mmultiscripts» «mi»element«/mi» «mo»§#160;«/mo» «mi»symbol«/mi» «/mstyle» «/math»

    «math xmlns=šhttp://www.w3.org/1998/Math/MathMLš»«mstyle mathsize=š24pxš»«mmultiscripts»«mo»§#160;«/mo»«mprescripts»«/mprescripts»«mn»20«/mn»«mn»44«/mn»«/mmultiscripts»«mi»Ca«/mi»«/mstyle»«/math»

    Remember that you need to include the mass number for this specific isotope. The atomic number is on the periodic table.
  5. Draw a modified Bohr diagram for an atom of chlorine-37.

    Remember that in an atom, the number of electrons is equal to the number of protons, which is the atomic number listed on the periodic table. Remember that electrons are placed two in the first energy level, a maximum of eight in the second energy level, and the remaining seven in the third energy level. Remember to place each electron separate, one per orbital for the first four electrons, then pair them up. Neutrons are determined by taking the mass number for this isotope and subtracting the atomic number. Protons and neutrons are placed in the nucleus.
  6. Write the isotope name for the isotope represented by this modified Bohr diagram.

  Interactive Activity

Element Builder © Gizmo


Background Information:

Please review this simulation to help you visualize and understand the structure of atoms and isotopes.

Please note: If you scroll down while in the Gizmo, you will see a list of questions. You DO NOT need to complete these questions. You are able to complete them for extra practice if you would like.


  1. Click on the play icon to open the virtual lab. Print students can access the Gizmo in the Online Resources for Print Students section of their online course.
  2. Click on “Show element name,” “Show element symbol,” “Show electron dot diagram,” and “Show Group and Period.”
  3. Add one electron to make a neutral particle of hydrogen-1. Note the structure and the element notation.
  4. Click on the play button.
  5. Add one neutron to make a neutral particle of hydrogen-2. Note the structure and the element notation.
  6. Add one more neutron to make a neutral particle of hydrogen-3. Note the structure and the element notation.
  7. Create a stable structure for helium-4 by adding one proton and one electron. Note the structure and the element notation.

    What do you need to do to create an isotope of helium-4?

    Add or remove one or more neutrons.
  8. Create a stable structure for lithium-6 by adding one proton, one electron, and one more neutron. Note the structure and the element notation.

    Is lithium-6 the most abundant isotope?

    No, lithium-7 is the most abundant isotope.
  9. Create a stable structure for fluorine-19. Note the structure and the element notation.
  10. Create a stable structure for aluminum-27. Note the structure and the element notation.
  11. Click on the analysis tab to complete the analysis questions.
  1. What happens when more than two electrons are in an element’s structure?

    They move farther out to fill the next energy level that is available.
  2. Use the element builder to create the following isotopes and fill in the missing information from this table.

    Isotope Name
    		 magnesium-25 chlorine-35
    Number of Protons
    		 15
    Number of Neutrons 18
    Mass Number
    		 18 15
    Number of Electrons 8
    Group Number
    		 15
    Number of Valence Electrons 5
    Period Number
    		 2
    Number of Energy Levels 2

    Isotope Name
    		 magnesium-25 phosphorus-33 chlorine-35 oxygen-18
    		 nitrogen-15
    Number of Protons
    		 12 15 17 8 7
    Number of Neutrons 13 18 18 10 8
    Mass Number
    		 25 33 35 18 15
    Number of Electrons 12 15 17 8 7
    Group Number
    		 2 15 17 16 15
    Number of Valence Electrons 2 5 7 6 5
    Period Number
    		 3 3 3 2 2
    Number of Energy Levels 3 3 3 2 2

  Organization of the Periodic Table

Did you know that Dmitri Mendeleev is often referred to as the Father of the Periodic Table?

B2.43 Dimitri Mendeleev
Dimitri Mendeleev, a Russian chemist, recognized that all of the known elements at the time created a pattern. It was such a distinct pattern that Mendeleev actually predicted the discovery of elements that would fit into the missing places. Over time, those elements were discovered and they did have the properties that Mendeleev predicted they would have.

The term periodic means at regular intervals; so the elements are arranged in such a way on the periodic table that elements that have similar atomic structure and characteristics create repetitive patterns.

  Did You Know?

Which element displays the most metallic character? Did you think of copper or iron? These are common metals, but they do not actually contain the most metallic character of all the elements. Watch this video to learn which element is considered the “most metallic” and which element is considered the “most non-metallic.”

Learn More

B2.44 Periodic table
The modern periodic table is organized in increasing atomic numbers.

  • Horizontal rows are known as periods and relate to the number of energy levels in an atom.
  • Vertical columns are known as groups or families and relate to the number of valence electrons, which are the electrons located in the outer most energy level, or valence energy level.
  • For groups 1, 2, and 13 to 18, the last digit of the group number is the same as the number of valence electrons for that element. (Valence electrons will become very important when we discuss compound formation in the next section.)

For example, if we study the modified Bohr diagrams for lithium, potassium, aluminium, and argon and relate their structures to their placement on the periodic table, we will see clear trends emerge.

Element lithium potassium aluminum argon
Modified Bohr Diagram
Group Number
 1 1 13 18
Number of Valence Electrons
 1 1 3 8
Period Number
 2 4 3 3
Number of Energy Levels
 2 4 3 3

These patterns can be used to predict the atomic structure of elements without needing to model it with a modified Bohr diagram. These patterns are also important to note when we start to investigate chemical reactivity, such as group 1 alkali metals being very chemically reactive while group 2 alkaline-earth metals are less reactive. Group 18 noble gases are stable, whereas group 17 halogens are very reactive, especially with alkali metals. These concepts will be addressed later on in this unit; but do you have a theory based on what you have learned so far?

The staircase line separates metals on the left from non-metals on the right, with the exception of hydrogen, as it is a non-metal.

These categories are in place based on shared characteristics between the elements that are classified as metals and those that are classified as non-metals. Further categorization is given to eight elements that border the staircase line. These elements, identified as metalloids, have some metallic properties and some non-metallic properties.

Metals Non-metals Metalloids
  • almost all solids at SATP (room temperature and pressure)
  • shiny
  • good conductors of electricity and heating
  • malleable
  • ductile
  • almost all solids at SATP (room temperature and pressure)
  • dull
  • poor conductors of electricity and heating
  • brittle
  • not ductile
  • all solids at SATP (room temperature and pressure)
  • some are shiny and some are dull
  • some conduct electricity
  • poor conductors of heat
  • brittle
  • not ductile

  Watch This

Alkali Metals in Water – Chemical Elements: Properties and Reactions @ YouTube OpenLearn from the Open University 


This video will illustrate how elements of one group have similar properties.

  Special Groups on the Periodic Table


B2.44 labelled periodic table

  Digging Deeper?

© Wikimedia Commons
B2.54 alternate periodic table

There are many different formats for the periodic table: spiral, 3-D Alexander model, etc. You can see many different examples on this wikimedia site or you can download and create your own 3-D Alexander model to have on your desk!

These are all soft, silvery metals that are very reactive with water and other substances. All atomic structures contain one valence electron. These elements are so reactive that they are not found in nature as pure elements but are quite common and abundant in ionic compounds bonded to halogens


© Wikimedia Commons
B2.45 Sodium metal
B2.46 Modified Bohr diagram of sodium

These are also silvery metals, but they are not as soft or as reactive as alkali metals. However, they do react easily with oxygen from the air to form an oxide coating. All atomic structures contain two valence electrons.


© Wikimedia Commons
B2.47 Magnesium metal
B2.48 Modified Bohr diagram of magnesium


These are reactive non-metals that all have seven valence electrons. These elements are so reactive that they are not found in nature as pure elements but are quite common and abundant in ionic compounds bonded to alkali metals.


© Alamy.com
B2.48 halogen gases
B2.49 Modified Bohr diagram of fluorine


These are very stable (unreactive) elements. They are stable because these atoms containing eight valence electrons, or in other words, the valences are full of electrons.


B2.50 Neon sign
B2.51 Modified Bohr diagram of neon


These are located in the centre of the periodic table. These are metals that are fairly stable, chemically speaking, with various physical appearances.


© Wikimedia Commons
B2.52 penny
© Wikimedia Commons
B2.53 silver box








  Watch This

How the Elements Got Their Names @ YouTube It’s Okay To Be Smart 


Every wonder why the symbol for sodium is Na or for gold is Au? This fun video looks at how different elements got their names and symbols.


  Read This

Please read pages 29 to 33 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on the design and layout of the periodic table. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. Which alkali metal will have five energy levels in its modified Bohr diagram?

    Rubidium. Alkali metals are group 1 elements, and having five energy levels means the element is located in the fifth period.
  2. Which energy level will be the valence energy level for a modified Bohr diagram of selenium? How many electrons will be in the valence energy level of a modified Bohr diagram of selenium?

    The fourth energy level. Selenium is located in the fourth period. Selenium will have six valence electrons because it is in group 16, as the last digit of the group number is equal to the number of valence electrons.
  3. What element has three valence electrons in its third energy level?

    Aluminum. The number of valence electrons is the last digit of the group number; so three valence electrons means the element is located in group 13. Third energy level means the element is in period 3.
  4. Which halogen will have two energy levels in its modified Bohr diagram?

    Fluorine. Group 17 is classified as halogens. Fluorine is located in period 2, as the number of energy levels is directly related to the period number.
  5. A student made the following statements about the element beryllium:
    1. It’s modified Bohr diagram will have two valence electrons.
    2. It will be a non-conductor of electricity.
    3. It’s modified Bohr diagram will have two valence electrons.
    4. It can be classified as an alkaline-earth metal.

      Which statement is incorrect?

    B. It is a metal, so it will be conductive.

  Conclusion

 Characteristics of atoms are the foundation of chemistry.

B2.55 Stylized atom
All matter consists of particles called atoms. The following is a list of the basic characteristics of atoms and how this information is related to the organization of the periodic table.

  • Atoms consist of a nucleus, which contains protons and neutrons (together these particles are called nucleons), and electrons, which are located in a cloud outside of the nucleus.
  • An electron can potentially be found at any distance from the nucleus, but depending on its energy level, they exist more frequently in certain regions around the nucleus than others; this pattern is referred to as the atomic orbital. The orbitals come in a variety of shapes (these shapes are beyond the scope of Science 10).
  • Each electron has a negative charge and is 1/1 840 the size of a proton or neutron.
  • Protons and neutrons are about the same size.
  • Each neutron is neutral.
  • Each proton has a positive charge.
  • The number of protons (also known as its atomic number) determines the element.
  • Varying the number of neutrons results in isotopes. Isotopes are variations of a single element.
  • Elements are arranged in order of increasing atomic number on the periodic table. This arrangement leads to patterns on the periodic table, such as period number representing the number of energy levels and the last digit of the group number representing the number of valence electrons.
  • Elements in groups or families have similar chemical properties.

  Interactive Activity

ChemThink Atomic Structure Tutorial @ Simbucket/PBS https://quick.adlc.ca/chemthink3


Work through this online tutorial to review the structure of the atom and subatomic particles.


2.2 Assignment

Unit 2 Formative Assignment  

It is now time to complete the Lesson 2 portion of 2.2 Assignment. Click on the button below to go to the assignment page. Complete only the Lesson 2 portion at this time.

2.2 Assignment