2.1.3 Trends of Period 3 Elements: First Ionisation Energy

• The first ionisation energy (IE₁) 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
  • Eg. the first ionisation energy of Na is:

Na (g) → Na⁺ (g) + e^-

IE₁ Values of the Period 3 Elements

• There is a general increase in IE₁ across a period
  • The nuclear charge increases
  • The atomic radius decreases
  • There are stronger attractive forces between the nucleus and outer electrons
  • It therefore gets harder to remove any electrons

• Small ‘dips’ are observed between Mg - Al and P - S