Unit B Lesson 4: The Atom

 Learning Targets

Big Question: What is the history of our understanding the atom?
 We use chemicals for many purposes at school, at home, and at work. It is your responsibility to make sure you know how to work with each chemical safely.

At the end of this inquiry, you should be able to answer the following questions:

  • What did Democritus propose?
  • How is the "plum pudding" model of an atom different from the "billiard ball" model?
  • What did Rutherford discover about positively charged particles?
  • What subatomic particle did Rutherford name?
  • How did Niels Bohr improve Rutherford's model?
  • Who is responsible for naming the particles with no charge as "neutrons"?
  • In the quantum model, what subatomic particle moves around the nucleus in a cloud?
Page 116 to 120  in your text will help you answer these questions about the atom.


Introduction


Atoms are the basic building blocks of matter that make up common objects. A table, the air, even you are made of atoms!  In this lesson, learn how the atom was discovered.
Democritus was a Greek philosopher (470-380 BCE) who proposed some important ideas.

  1. Matter could be broken into tiny particle called atomos, which meant “uncuttable” or “indivisible” (modern “atoms”).

  2. By mixing various “atomos”, new materials can be produced with unique sets of properties

 
 


John Dalton

John Dalton (1766–1844), an English chemist, revived Democritus’ ideas and envisioned his “billiard ball” model because he thought of the tiny atoms as solid spheres.

  1. Elements are made of atoms.
  2. The atoms of an element are identical in their masses.
  3. Atoms of different elements have different masses.
  4. Atoms combine to form compounds only in small, whole number ratios such as 1:1, 1:2, 2:3 and so on.
  5. Atoms cannot be created nor destroyed nor divided into smaller particles.

Dalton was wrong about one thing: Atoms can be divided into smaller particles.



 
 

J. J. Thompson

J. J. Thompson (1856-1940), a British physicist, was the first scientist to show, using cathode ray tubes, that the atom can be broken further.  Thompson discovered the very small, very light, negatively charged particles.  Thompson called them corpuscles, but we now call them electrons.  He also proposed the existence of a positive particle.

Thompson proposed the Plum Pudding model.


He suggested that the atom was a giant sphere that was positively charged with negatively charged electrons embedded throughout.


 
 

Image courtesy Structure of an Atom (Rice OpenStax)

Ernest Rutherford

In 1909, Ernest Rutherford (1871- 1937), a British physicist, tested Thompson’s model of the atom. He fired alpha particles at a piece of gold foil that was only a few atoms thick.

Rutherford predicted that if one fired alpha particles at an atom, they would fly right through the foil with no deflection.

What he discovered is that most alpha particles will pass through but some will scatter in many directions.  He concluded that the positive charge must be in a small area called the nucleus.

Rutherford's Prediction


The Rutherford atomic model
Because most alpha particles pass through the atom, he concluded that most of the atom was empty space.  He deduced correctly that the positively charged particles were found in the nucleus (centre of the atom) and he called them protons.

Rutherford's Experimental Result
 
 

Niels Bohr

Niels Bohr (1885-1962), a Danish physicist, improved Rutherford’s model.  He proposed that electrons move around the nucleus (that contains the protons) in specific layers, or shells.
 
 
 

James Chadwick

Working with Rutherford, Chadwick (1891-1974), a British physicist, discovered particles with no charge, called neutrons.  Neutrons are found also in the nucleus.  Neutrons have a mass similar to protons.  Electrons have a mass of only 1/1837th of a proton or neutron.

The neutron's main function in the nucleus is to use its “strong force” to prevent the positively charged protons from repelling each other.

 
 

Figure 1 – The electron cloud around the nucleus of a helium atom. There are two electrons in the cloud surrounding the helium nucleus.
Quantum Model

  • The quantum model has electrons moving around the nucleus in a cloud (yellow cloud in Figure 1). The cloud is three-dimensional, like a round balloon.
    • The cloud of an atom of helium is very large compared to the nucleus, but only two electrons are found in that cloud space.

  • The nucleus, found at the centre of the cloud, is very tiny compared the the cloud itself (the enlarged dot at the centre of the cloud in figure 1).
    • The nucleus is a very small part of the atom
    • The nucleus holds the protons and neutrons.

  • The helium atom shown in figure 1 is about 1 Angstrom in diameter. For comparison, a human hair is about 1 million angstroms thick!

  • The quantum model is most accurate model of the atom although scientists continually study the atom to improve this model.

Figure 2 – The electron cloud of a hydrogen atom. The brighter the area, the more likely the electron will be found there.

 Watch

The following video is an excellent overview of how theories of matter have changed over time.




Interactive


Atomic Model

  1.  Watch the BrainPOP video, "Atomic Model", to reinforce your knowledge of the history of the atoms.

    Click here to watch the video. Try some of the other interactive activities when you have finished.

    You will need a username and password to access the video:
    • Username: 0099
    • Password: students
  2.  Complete the Atomic Model quiz.  Select the “classic” quiz option.  Remember to check your answers at the end!  Click here to access the quiz.

Interactive


Atoms

  1. Watch the BrainPOP video, "Atoms", to reinforce your knowledge of the structure of atoms.

    Click here to watch the video. Try some of the other interactive activities when you have finished.

    You will need a username and password to access the video:
    • Username: 0099
    • Password: students
  2.  Complete the atoms quiz. Remember to check your answers at the end!  Click here to access the quiz.