The first ionisation energy is the energy required to remove one electron from each atom in one mole of gaseous atoms of an element to form one mole of gaseous +1 ions.
Na (g) → Na+(g) + e- +496kJmol-1
The atomic radius has a large effect. The greater the distance of the outer electrons from the nucleus the less the nuclear attraction.A larger Nuclear charge increases the ionisation energy, the more protons there are, the greater the nuclear attraction.
The repulsion of the outer electrons by inner shell electrons is called the shielding effect and reduces the nuclear attraction.