1. Models of the Atom
Remember: models are used to help us to understand what we cannot readily see…they can change as we continue to learn.

2. Bohr Model Electrons travel in orbits/paths
These orbits have fixed energies…we can call them energy levels.
A quantum is the amount of energy to move from one to another energy level.
There are different amounts of energy needed to change to different levels.

3. Quantum Mechanical Model
Determines how much energy an electron can have and how likely it is to be found in various locations around the nucleus.
The realization is that we can only give a probability of where the electrons are…this probability of location is now called an atomic orbital.

4. Principle Energy Levels
The first number is the energy level.
The second number is the sub-shell; these are named s, p, d, and f. The number of available sub-shells increases as the energy level increases. For example, the first energy level only an s sub-shell while the second energy level contains an s sub-shell and a p sub-shell.

5. Electron Capacity in Sub-shells
The number in superscript is the number of electrons in a given sub-shell. Each sub-shell can only hold a particular number of electrons. These electrons are held in pairs.
S: has capacity for 2 electrons
P: has capacity for 6 electrons
D: has capacity for 10 electrons
F: has capacity for 14 electrons

6. Arranging the Electrons
Aufbau Principle: electrons occupy the lowest energy orbitals first. See fig. 5.7, 133.
Pauli Exclusion Principle: an atomic orbital may describe (locate) either one or two electrons, and they must have opposite spins; denoted with ↑ or ↓.
Hund’s Rule: one electron enters each orbital until all are occupied with one of the same spin direction before the second electron, of opposite spin, enters the first orbital.
There are a few exceptions: pg. 136.

7. QMM, continued Quantum Numbers
n = 1, 2, 3, ...., denoting energy;
l = 0, 1, ..., n-1, denoting angular momentum
m = -l, ...., l, denoting orientation (the "magnetic quantum number"),
s = -1/2, 1/2, denoting spin (m s is also used in place of s)