How would you explain the fact that the first ionisation enthalpy of sodium is lower than that of magnesium but its second ionisation enthalpy is higher than that of magnesium?

Explanation

Atomic radius is one half of the distance between the nuclei of two covalently bonded atoms of the same element in a molecule. In case of metals, the atomic radius is called metallic radius. It corresponds to one half of the distance between two adjacent atoms in a crystal lattice. Ionic radius means the size of the ion i.e., a cation or anion. This gives the effective distance from the nucleus of the ion upto which it has an influence in the ionic bond. The size of the cation is always smaller than that of the parent atom while the size of the anion is always larger than that of the parent atom.