Why do ionisation energies typically increase upon going across a period yet decrease upon going down a group?

‘Ionisation energy’ is the term used to describe the amount of energy it takes to remove an electron from an atom to form a positively charged ion. It is usually represented by the unit kJ/mol, meaning it describes the amount of energy (in kJ) it takes to remove one electron from each atom in one mole of an element.

The interplay between electrons and the nucleus in atoms is the main driving force behind chemical reactivity – and ionisation energies are no exception. As one goes across a period, the number of protons in the nucleus increases, giving each element a stronger nuclear charge going across the periodic table. This higher nuclear charge attracts the electrons orbiting the atom, pulling them closer and closer to the nucleus, thus making electrons more difficult to remove from the atom as one goes across a period – giving higher ionisation energies as you go across a period.

However, the number of electron shells surrounding the nucleus increases as you go down a group. This increasing number of electrons in each shell helps shield the outer electrons from nuclear charge, making the outer electrons easier to remove as one goes down a group in the periodic table – meaning ionisation energies typically decrease going down a group.