Electron configuration

1. Aufbau principle

Electrons fill the lowest-energy subshell first. Follow the diagonal arrows in the filling-order diagram: 1s, 2s, 2p, 3s, 3p, 4s, 3d… That's why potassium's 19th electron goes into 4s, not 3d.

2. Pauli exclusion principle

Each orbital (one box) holds at most two electrons, and they must spin in opposite directions, shown as ↑↓. So an s subshell holds 2, p holds 6, d holds 10 and f holds 14.

3. Hund's rule

Inside a subshell, electrons spread out one per box before any pair up. Step from boron to neon and watch the three 2p boxes each take one ↑ before the first ↓ arrives.

How to write an electron configuration

1. Find the atomic number: that's how many electrons a neutral atom has.
2. Fill subshells in the filling order, up to their limit (s 2, p 6, d 10, f 14).
3. Write each subshell with its electron count as a superscript: 1s² 2s² 2p⁴ for oxygen.
4. Check: the superscripts should add up to the atomic number.
5. For the short form, replace the inner part with the previous noble gas: [He] 2s² 2p⁴.

Exceptions to the filling order

19 measured elements don't follow it exactly, including chromium, copper, niobium, molybdenum, ruthenium, rhodium. Chromium is [Ar] 3d⁵ 4s¹ rather than [Ar] 3d⁴ 4s², because a half-full d subshell is extra stable.