Writing electronic configurations

By the end of this lesson

By the end you can write the full and shorthand electronic configuration of any atom or ion from H to Kr (chromium and copper included) and explain the filling order using energy and repulsion.

Three filling rules

Lowest energy first. Fill sub-shells in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. The ground state is the lowest-energy arrangement.
Two per orbital, maximum, and a pair must have opposite spins.
Spread out before pairing. In a sub-shell, electrons go into separate orbitals before any orbital gets a second electron.

Rule 3 is about inter-electron repulsion: electrons are all negative, so two in the same orbital repel more than two in separate orbitals of the same energy. Spreading out gives a lower total energy.

Reading and writing the notation

In 2p⁶, the 2 is the shell, the p is the sub-shell and the ⁶ is the number of electrons in it.

Full: list every sub-shell from 1s. Sodium: 1s² 2s² 2p⁶ 3s¹.
Shorthand: replace the inner part with the previous noble gas in square brackets. Sodium: [Ne] 3s¹.

[He] = 1s², [Ne] = 1s² 2s² 2p⁶, [Ar] = 1s² 2s² 2p⁶ 3s² 3p⁶.

Worked example: Iron, Z = 26

Write the full and shorthand electronic configurations of iron, and give the number of electrons in each shell.

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Quick check

Multiple choiceQuestion 1 · Recall

CalculateQuestion 2 · Recall

CalculateQuestion 3 · Apply

Answer the 3 questions above to continue.