1. Niels Bohr
The Bohr Model

2. J. J. Thomson discovered the electron using a cathode ray tube
J.J. Thomson discovered the electron using a cathode ray tube. The fact that the ray was deflected by a magnet meant the beam was made of particles. The fact that it was attracted to the positive electric charge meant that these electrons were negatively charged (opposites attract).

3. Expected Result
Actual Result
Ernest Rutherford performed his famous gold foil experiment. Based on Thomson’s model, he expected all of the alpha particles to fly through the gold foil. When some of them unexpectedly bounced back, he realized that there must be a dense region at the center of the atoms: the nucleus. However, he still did not understand how the electrons traveled around the nucleus.

4. Ernest Rutherford’s student, James Chadwick, discovered the presence of neutrons within the nucleus. This was an important discovery that broadened our understanding of the atom.
James Chadwick
Ernest Rutherford

5. Watch the short video over Bohr’s Model of the Atom.

6. Carbon
Helium
Oxygen
Sodium
Niels Bohr, a Danish physicist, went on to improve atomic theory even further. He proposed that electrons were in fixed orbits at different distances from the nucleus. These orbits represented different energy levels.

7. Electron Cloud Model of Helium
Bohr Model of Helium
Electron Cloud Model of Helium
It turns out Bohr was wrong about electrons being in fixed orbits, but he was correct about them occupying different energy levels. Even though he was wrong, we still often use his model to depict atoms because it helps make the idea of energy levels easier to understand. Today we will explore these energy levels further.

8. The current model of the atom is known as the Electron Cloud Model
The current model of the atom is known as the Electron Cloud Model. To truly understand the placement of the electrons in these clouds you need very complex math, but it is important that you know that electrons do not travel in fixed orbits. There were many scientists involved in our current understanding of the atom.

9. Bohr was able to study the different energy levels of atoms by analyzing the light that was emitted from atoms.
Bohr understood that as atoms absorb energy, electrons can move up an energy level. These electrons at a higher energy level are in the excited state.
When these electrons drop back down into their ground state, they release energy in the form of light. This makes them glow.

10. Bohr was able to calculate the difference in energy between the levels through his understanding of the speed of light and the relationship between the frequency and wavelength of light.
All forms of radiation, which includes all colors of light, travel at the same speed:
c = 3.00 x 108 m/s

11. The frequency and wavelength of all forms of radiation, including light, are inversely related. This means that as one goes up, the other comes down.
Because E.M. radiation all goes at the same speed, if you know the frequency, you can find the wavelength. (The opposite is also true.)

12. Calculate the frequency or wavelength of light in each of the following problems.
c = fλ
3.00 x 108 m/s = f (5.78 x 10-7 m)
f = x Hz
3.00 x 108 m/s = f
(5.78 x 10-7 m)
c = fλ
3.00 x 108 m/s = (6.8 x 1014 Hz) λ
λ = x m
3.00 x 108 m/s = λ
(6.8 x 1014 Hz)

13. Bohr also understood that the energy of a photon of light was directly related to its frequency. The higher the frequency of light, the greater the amount of energy. In the visible light spectrum, violet light has the highest frequency and the highest energy. Red light has the lowest frequency and the lowest energy.
Using the work of Max Planck and the constant he discovered, Bohr was able to determine the amount of energy being emitted from an atom in the form of light.

14. 1. The electron in the diagram drops from its excited state to its ground state. In the process, it emits a photon with a frequency of 6.65 x 1014 Hz. How much energy is in the photon of light?
4.41 x J
2. A photon of light with a wavelength of 5.28 x 10-7 meters is emitted from an atom as an electron travels from its excited state to its ground state. How much energy difference was there between the excited state and the ground state of the electron assuming all of the energy is contained in the photon of light?
3.77 x J