1. Electron Configurations Where the electrons are in the energy levels and orbitals. The configuration that requires the least energy is the most stable - called ground- state electron configuration. 3 specific rules are used to find an atom’s electron configuration: –Aufbau principle (German for build up) –Pauli exclusion principle –Hund’s rule

2. Aufbau Principle Says each electron occupies the lowest energy orbital available. An Aufbau diagram shows the orbitals in order from lowest to highest energy required Hints: –All orbitals within a sublevel have equal energy. (All 3 p sublevels are equal energy at any level.) –Energy sublevels within a principle energy level have different energies (s<p<d<f). –Generally energy increases with n but there is some overlap between sublevels.

3. Pauli Exclusion Principle Each electron in an atom is spinning. There are two possible directions for an electron to spin. An orbital can only contain two electrons if they have opposite spins. The two spins are represented by an arrow pointing up and an arrow pointing down.

4. Hund’s Rule Because electrons repel each other, each orbital within an energy level will have one electron before any orbital has a second electron in it. Can be thought of as the chemistry “bus” rule – all of the double seats on a bus fill up with single riders before anyone doubles up.

5. Orbital Diagrams One way to represent an atom’s electron configuration. Includes a box for each of the atom’s orbitals. Each box can contain an up arrow and a down arrow. Each box is labeled with the principle quantum number (energy level) and sublevel.

7. Electron Configuration Notation Shows each energy level and sublevel followed by a superscript indicating the number of electrons in it. Does not show the orbital distribution of electrons like an orbital diagram does. For example, carbon in its ground state would be represented: 1s 2 2s 2 2p 2

8. Valence Electrons The electrons in the outermost energy level – includes all orbitals. These electrons determine an atom’s chemical behavior and properties. These electrons form chemical bonds with other atoms.

9. Noble Gases Noble gases always have a full s and a full p orbital for their valence electrons So the electron configuration for a noble gas can be represented by: [Ne] Instead of 1s 2 2s 2 2p 6

10. Noble Gas Configuration Used to shorten any element’s electron configuration You find the noble gas from the previous period then write the configuration for the outer energy level: Na = 1s 2 2s 2 2p 6 3s 1 = [Ne]3s 1 The noble gas shows what’s in the inner energy levels and the rest is the valence electrons

11. Try It 1.Write the electron configuration for bromine then rewrite it in noble gas configuration: 2.Repeat #1 for rubidium:

12. Electron Dot Configuration Even quicker than noble gas configurations. Shows only the elements symbol and a dot for each valence electron it has Examples of Rb and Br: