2.2 Electron configuration

Noble gas (or condensed) electron configuration

Example for Carbon it is [He] 2s² 2p²

Orbital diagram

Is just the arrows (up and down) that corresponding to the configuration. Just remember the numbers of orbitals (boxes) for each sublevel (s, p, d, f)

The order (from lowest to highest energy) of the different shells
note that each shell is an energy level that is quantized, meaning there is NO in-between such as level 1.5, 2.3, 4.8… Electron has to be in an energy level!

Example: write the electron configuration for Fe and Fe²⁺

Each energy level (n=1, 2, 3...) has a maxium number of electrons 2n²

n = 1  --> max 2 electrons

n = 2  --> max 8

n = 3  --> max 18

apply a bit a math the equation is 2n²

(or logically, each orbital has 2 electrons hence 2 x something. The geometric sequence for the number of orbital is 1, 1+3, 1+3+5, ... so n²)

Each orbital has a specific shape

Exceptions and their ions

Memorize a few exceptions in filling the sublevels for electron configuration

Cr = [Ar] 3d⁵ 4s¹ --> Cr¹⁺ = [Ar] 3d⁵

Cu = [Ar] 3d¹⁰ 4s¹ --> Cu¹⁺ = [Ar] 3d¹⁰

Electron configuration rules

1. Pauli exclusion principle: two electrons in the same orbital have to have opposite spin (ie. In a box one arrow point up, the other point down)
2. Hund's rule: electrons will fall into empty orbitals of the same energy before electrons begin to pair up into the same orbital (ie. Fill all the box up first then fill in the second electron)
3. Aufbau principle: electrons will fall into lower energy level first before going to the higher energy level (ie. The order 1s, 2s, 2p, 3s, 3p, 4s…)