Electron configuration: Ionisation energies.

The idea

The first ionisation energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms, forming one mole of gaseous 1⁺ ions. It is always endothermic because work is done against the electrostatic attraction between the nucleus and the electron. Three factors control its size: nuclear charge, which increases attraction; atomic radius, since a more distant electron is held less strongly; and shielding by inner shells, which reduces the effective pull. Successive ionisation energies always rise, partly because each electron after the first is pulled away from an ion that is already positive and smaller, so the remaining electrons are held more tightly.

What earns the marks

• Removal of one electron from each atom in one mole of gaseous atoms.
• The product described as one mole of gaseous 1⁺ ions.
• Naming nuclear charge, atomic radius and shielding as the controlling factors.
• Explaining a rise in successive values by removal from an increasingly positive ion.

Common mistakes and how to put them right

• Omitting the gaseous state from the definition or from the equation.

• Defining it as the energy to remove all the electrons rather than one per atom.

• Describing ionisation as exothermic because an ion is formed.

• Referring to one atom rather than one mole so the quantity has no molar units.

Exam tips

• Learn the definition as a single sentence containing the words one mole, gaseous and 1⁺ ions.
• Always add state symbols to ionisation equations; they carry a mark of their own.
• When comparing two values, mention all three factors and say which one dominates.