1. Models of the Atom

2. Model vs. Theory
A Model is a human construct to help us better understand real world systems.
A Theory is an explanation of the natural world that can incorporate laws, hypotheses and facts.
It is testable, and can be refined or rejected. A model is used to explain a theory

3. Empedocles: Greece 450 B.C.
Everything is made up of these four elements
Different types of matter are combinations of these basic elements
Based on philosophy, not experiment
Model accepted for 2000 years.

4. Democritus: Greece 400 B.C.
Matter is made of tiny indivisible particles called atoms
Different elements are made of different atoms
All matter is made up of atoms and empty space

5. Dalton: England 1650 All matter is made of atoms too small to see.
Dalton is the reason we now use PIASM
Each element has its own kind of atom with characteristic properties
Compounds created when atoms of different elements link to form molecules
Atoms cannot be created or destroyed in a chemical reaction
Billiard Ball Atom

6. J.J. Thomson: England 1904
Discovered evidence of particles within the atom
His “claim to fame” was determining that atoms contain negatively charged particles called electrons
Thought that electrons were embedded in a positive sphere so that the resulting atoms are neutral or uncharged
Raisin Bun Model

7. Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle.
Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

8. Rutherford: New Zealand 1911
Positive centre called the nucleus
Protons are positively charged particles in the nucleus
Surrounded by mostly empty space containing rapidly moving negative electrons
At 5’9”, my nearest e- would be 10 km away!

9. How’d he figure that out!?
The Gold foil experiment
Used a thin film of gold foil to prove that matter was mostly empty space, the electron should have gone straight through…
It didn’t: some bounced back!
Matter was therefore mostly empty space, but at its core had a tiny dense mass called the nucleus

10. Try it Yourself!
In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

11. The Answers
Target #1
Target #2

12. There was a problem with Rutherfords Model:
If opposites attract, why do the electrons not fly into the nucleus!?
This is where Bohr stepped in

13. Bohr: Denmark 1913 Electrons orbit the nucleus in shells
Only a certain number of electrons can be in each shell
Orbits represent different energy levels of electrons
BUT: electrons cannot exist between energy levels. What happens?

14. Light is a particle?
If light can have particle properties, and is a form of energy, maybe since electrons are particles with mass, maybe they can have energy properties
Since electrons cannot exist between energy levels, they only “jump” them, and get or give off energy.
We will get back to this in a few days!

15. Chadwick: England 1932
Through experimentation, James Chadwick suggested the existence of neutrons, particles with no charge in the nucleus
These particles have the same mass as the protons

16. Plank, Schrodinger, Heisenberg: 1926
Electrons create standing waves
Orbitals can be described as electron density clouds
The densest area of a cloud is where the greatest probability of finding the electron is

17. Table 1, p. 26

18. Atomic numbers, Mass numbers
Elements are often symbolized with their mass number (A) and atomic number (Z)
E.g. Oxygen: O 16 8
Z = # of protons = # of electrons
A - Z = # of neutrons
Calculate # of e–, n0, p+ for Ca, Ar, and Br

19. Atomic
Mass p+ n0 e– Ca 20 40 20 20 20 Ar 18 40 18 22 18 Br 35 80 35 45 35

20. Isotopes
Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.
Isotope
Protons
Electrons
Neutrons
Nucleus
Hydrogen–1
(protium) 1
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium) 2

21. Composition of the nucleus
Atomic Masses
Atomic mass is the average of all the naturally isotopes of that element.
Carbon =
Isotope
Symbol
Composition of the nucleus
% in nature
Carbon-12
12C
6 protons
6 neutrons
98.89%
Carbon-13
13C
7 neutrons
1.11%
Carbon-14
14C
8 neutrons
<0.01%