Ionisation energies and electron shellsEdexcel International A Level Chemistry: Revision notes

The first ionisation energy is the energy required to remove one electron from each atom in one mole of gaseous atoms, forming one mole of gaseous 1+ ions:

X(g) → X⁺(g) + e⁻

The second ionisation energy is the energy to remove one electron from each ion in one mole of gaseous 1+ ions:

X⁺(g) → X²⁺(g) + e⁻

The third ionisation energy is defined in the same way for X²⁺(g) → X³⁺(g) + e⁻. Units are kJ mol⁻¹. All ionisation energies are endothermic, because energy must be supplied to overcome the attraction between the nucleus and the electron.

Three factors control how strongly an electron is held:

• Nuclear charge: more protons attract the electron more strongly, increasing the ionisation energy.
• Distance and shielding: an electron in a shell further from the nucleus, with more inner shells between it and the nucleus, is held less strongly.
• Sub-shell: an electron in a higher-energy sub-shell (p rather than s in the same shell) is easier to remove. Electrons paired in an orbital repel each other and are also easier to remove.

Removing electrons one after another gives successive ionisation energies. Each one is larger than the last, because the electron is removed from an ion that is more positively charged and smaller.

A large jump occurs when the next electron comes from a new, inner shell, which is closer to the nucleus and less shielded.

Example: element X has values 578, 1817, 2745, 11 577, 14 842 kJ mol⁻¹. The big jump comes after the third, so three electrons are in the outer shell and X is in Group 3.

Sodium's successive values (kJ mol⁻¹) are 496, 4562, 6910, 9543, 13 354, 16 613, 20 117, 25 496, 28 932, 141 362, 159 076.

• Big jump after the 1st: one electron in the outer shell (Group 1).
• Values 2 to 9 rise gradually: eight electrons in the second shell.
• Big jump after the 9th: the 10th and 11th electrons are in the first shell, giving 2,8,1.

A graph of the logarithm of each value against the number of electrons removed shows the shells as clear steps.

Down a group the first ionisation energy decreases: the outer electron is in a shell further from the nucleus with more shielding, and this outweighs the increased nuclear charge.

Across a period it generally increases: the nuclear charge increases while the electrons are added to the same shell, so the atomic radius decreases.

Two small dips in Period 3 show sub-shells:

• Al lower than Mg: Al's outer electron is in a 3p sub-shell, higher in energy than the 3s of Mg, and shielded by the 3s electrons.
• S lower than P: in S, two electrons are paired in one 3p orbital. Their repulsion makes one easier to remove.

The same pattern occurs in Period 2: B is lower than Be, and O is lower than N.