Trends of Period 3 Elements: First Ionisation Energy (AQA A Level Chemistry): Revision Note

Trend: First Ionisation Energy

• The first ionisation energy (IE) is the energy required to remove one mole of electrons from one mole of atoms of an element in the gaseous state to form one mole of gaseous ions

  • For example, the first ionisation energy of sodium is:

Na (g) → Na⁺ (g) + e^-

First ionisation energies of the Period 3 elements

• There is a general increase in first ionisation energy across a period

• Across a period, the effective nuclear charge increases because the number of protons increases, while shielding remains approximately constant

  • As a result, the atomic radius decreases

• The stronger attraction between the positively charged nucleus and the outer electron makes it more difficult to remove the electron, so the first ionisation energy increases

• However, small decreases (or “dips”) are observed between magnesium and aluminium, and between phosphorus and sulfur

• The dip between Mg and Al occurs because

  • Al is [Ar]3s²3p¹ and Mg is [Ar]3s²

  • The outer electron removed from aluminium is in a higher-energy 3p subshell, which is slightly easier to remove than a 3s electron

• The dip between P and S occurs because:

  • S is [Ar]3s²3p⁴, and P is [Ar]3s²3p³

  • Sulfur has a pair of electrons in one 3p orbital, and electron–electron repulsion makes it easier to remove one of the paired electrons compared with phosphorus, where the 3p electrons are unpaired