Electron configuration: filling the orbitals

Hydrogen has one electron, sitting in the lowest-energy 1s orbital. Iron has 26, spread across a whole sequence of orbitals. They are not placed at random: from the atomic number alone you can predict every electron’s “seat” — and the shape of that seating chart is exactly the shape of the periodic table.

1. Aufbau (lowest energy first): electrons occupy the lowest-energy empty orbital. In many-electron atoms orbital energy rises roughly with $n+l$; for equal $n+l$ the smaller $n$ fills first.
2. Pauli exclusion: an orbital holds at most two electrons, with opposite spins (↑↓).
3. Hund’s rule: orbitals of one subshell have equal energy, so electrons first spread out singly with parallel spins before any pairing.

Add electrons one at a time with the arrow buttons. Watch three things: the three p boxes each get one ↑ before any pairing (Hund’s rule); every completed p subshell lands on a noble gas; and the lit-up region of the periodic table grows block by block — s, p, d, f.

Inner electrons barely take part in chemistry, so we abbreviate them with the preceding noble gas: iron is $[\mathrm{Ar}]\,3d^6\,4s^2$. Whatever sits outside the brackets are the valence electrons that do the bonding. Elements in one group share a valence configuration — that is why they behave alike.

Set the explorer to chromium (Z = 24). The $n+l$ rule predicts $[\mathrm{Ar}]\,3d^4\,4s^2$, but spectroscopy finds the ground state $[\mathrm{Ar}]\,3d^5\,4s^1$. Copper likewise is $3d^{10}\,4s^1$, not $3d^9\,4s^2$.

The reason: 3d and 4s are very close in energy, and electrons with parallel spins gain extra exchange stabilisation. A half-filled ($d^5$) or filled ($d^{10}$) subshell maximises it — enough to pay for moving one electron from 4s to 3d. The $n+l$ rule is an approximation; experiment is the referee.

When a cation forms, electrons leave the subshell with the highest principal quantum number — not the one filled last. So Fe → Fe²⁺ removes the two 4s electrons: $[\mathrm{Ar}]\,3d^6$. Once the atom carries a positive charge, 3d drops below 4s in energy. Anions simply keep filling by the aufbau order: $\mathrm{Cl^-}$ is $[\mathrm{Ne}]\,3s^2\,3p^6$, isoelectronic with argon.