Lesson 2 Matter

  Bohr's Atomic Theory

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.

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B2.23 Fireworks

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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.

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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?

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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.

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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

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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.
  
  

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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.

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  Practice Questions

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

   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.
   
   
   Check your work.
   

   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?
   
   
   Check your work.
   

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