1. Atomic Theory – Bohr & Chadwick

2. Let’s talk about Niels Bohr
The Bohr model of the atom
Recall: The Rutherford model
positive nucleus
negative electrons orbiting the nucleus

3. Niels Bohr (1922)
Predicted that electrons are located in different “energy levels”

4. Features of Bohr’s model:
electrons don’t just orbit randomly
they are restrict to fixed orbits
each orbit represents a different level of energy
energy is quantized – i.e., electrons at each energy level possess a specific amount of energy
Based his model on the line spectrum of hydrogen

5. The Line spectrum of Hydrogen
Where do these lines come from??

6. Hydrogen’s electron absorbs EM energy Jumps to a higher energy level
The electron is in an “excited state”
unstable

7. As the electron falls back down to its normal stable state (“ground state”), it emits the energy it absorbed.
Emitted energy is visible as its line spectrum.

8. Observations Multiple lines but only one electron
The electron can absorb more than one amount of energy
Spectrum is not continuous
The electron can absorb only specific amounts of energy

9. Since each energy level is fixed, only certain amounts of energy can be absorbed or emitted. Link here:

10. Bohr’s Model of the Atom

11. Bohr’s Model of the Atom
Electrons can move to higher levels if energy is absorbed by the electron – the electron is temporarily unstable.
Unstable electrons fall back down to their “normal” or lower level and release the stored energy as light of a specific wavelength (color).

12. Bohr model Limitations
Was not useful for predicting the line spectra of other elements
BUT was a good starting point for modern atomic theory

13. Our classmates are going to present information about electrons/energy levels and more and that is how we will learn more on this timeline of the atomic theory.

14. James Chadwick (1932) Discovered Neutrons
Neutral particles located in the nucleus
Bombarded different materials (paraffin, helium, etc.) with high energy gamma photons.
Noticed that a particle with no charge was ejected

15. Unit 2 - Lecture 1: Structure of the Atom
James Chadwick – The Neutron
In the nucleus with the protons are particles of similar mass but no electrical charge called neutrons.
Electrons (-) are now outside the nucleus in quantized energy states called orbitals. (From Niels Bohr and quantum mechanics)
The positively charged particles in the nucleus are protons. + n n
Structure of the atom after Chadwick (1932)

16. What we now consider facts about the atom
As provided by MANY experiments and consistent results…

17. Unit 2 - Lecture 1: Structure of the Atom
proton (+)
neutron
electrons - responsible for the volume and size of the atom, negatively charged
10-10 m
nucleus - responsible for the mass of the atom, positively charged
10-14 m

18. Fill in your “Evolution of Atomic Theory” Timeline