Electron configuration, ionisation energy and periodicity

What you’ll learn

• Write configurations using shells, subshells and orbitals.
• Explain ionisation-energy trends and exceptions.
• Relate successive ionisations to valence electrons.

Understand the idea

An orbital holds at most two electrons with opposite spins. The s, p and d subshells contain 1, 3 and 5 orbitals respectively. Orbitals in one subshell are filled singly with parallel spins before pairing. An s orbital is spherical; each p orbital has two lobes. For neutral atoms the usual filling order includes 4s before 3d; chromium and copper have the familiar $3d^54s^1$ and $3d^{10}4s^1$ configurations.

First ionisation energy removes one electron from each atom in a mole of gaseous atoms. It is endothermic because attraction to the nucleus must be overcome. Across a period, increased nuclear charge with similar shielding generally raises ionisation energy. Down a group, extra shells and shielding generally outweigh increased nuclear charge.

Subshell and pairing changes explain local exceptions: an outer p electron can be easier to remove than an s electron in the same shell, and a paired p electron experiences extra repulsion. A large jump in successive ionisation energies signals removal from an inner shell after valence electrons have gone. When forming transition-metal cations, remove 4s electrons before 3d electrons.

Periodic melting trends depend on structure as well as electron configuration. Across Period 3, metals give way to the giant covalent silicon network and then simple molecular substances and monatomic argon. Do not explain every melting-point difference using nuclear attraction alone.

Key formulae

$$\mathrm{X(g)\rightarrow X^+(g)+e^-}$$

$$\mathrm{X^+(g)\rightarrow X^{2+}(g)+e^-}$$

$$\mathrm{Na}:1s^22s^22p^63s^1$$

$$\mathrm{Fe^{2+}}:[\mathrm{Ar}]3d^6$$

Worked example

Explain why aluminium has a lower first ionisation energy than magnesium despite its higher nuclear charge.

1. Magnesium ends in $3s^2$; aluminium ends in $3s^23p^1$.
2. Aluminium loses a 3p electron, whereas magnesium loses a 3s electron.
3. The 3p electron is in a higher-energy subshell and is more shielded, so it is easier to remove despite the increased nuclear charge.

Answer: The change from removal of a 3s electron to removal of a 3p electron causes the decrease.