Unit 1 · Lesson 2

Electron Configuration

Energy Levels & Subshells

Electrons occupy energy levels (shells) labeled \(n = 1, 2, 3, \ldots\). Each energy level contains one or more subshells: \(s\), \(p\), \(d\), and \(f\).

Each subshell contains a specific number of orbitals, and each orbital holds a maximum of 2 electrons:

Three Key Rules

Aufbau Principle: Electrons fill the lowest energy subshells first. The filling order is:

$$1s \rightarrow 2s \rightarrow 2p \rightarrow 3s \rightarrow 3p \rightarrow 4s \rightarrow 3d \rightarrow 4p \rightarrow \ldots$$

Pauli Exclusion Principle: Each orbital holds at most 2 electrons, and they must have opposite spins (one up, one down).

Hund's Rule: Within a subshell, electrons occupy empty orbitals first before pairing up. Think of it like bus seats -- everyone sits alone before doubling up.

Writing Electron Configurations

For Carbon (\(Z = 6\)), we place 6 electrons following the Aufbau order:

$$\text{C}: 1s^2\, 2s^2\, 2p^2$$

For larger atoms, we use noble gas shorthand. Instead of writing every subshell, start from the previous noble gas. For Sulfur (\(Z = 16\)):

$$\text{S}: [\text{Ne}]\, 3s^2\, 3p^4$$

This means sulfur has the same configuration as Neon (10 electrons), plus 6 more in the 3s and 3p subshells.

Practice Exercises