The minimum amount of energy which is required to remove an outermost electron from any isolated neutral gaseous atom is known as first ionisation energy. These are the following factors which effect ionisation energy.
(i) Ionisation energy `prop (1)/("Principal quantum number")`
(ii) Ionisation energy `prop Z_(eff)`
(iii) If orbitals are fully filled or half filled so stability will be more and ionisation energy will be high.
(iv) If penultimate electron will effectively shield the nucles, ionisation energy will be less and vice versa.
Which of the following has maximum Ist ionisation energy?

Options

A. `He`
B. `Mg`
C. N
D. `Na`

Answer

A