The amount of energy required to remove the most loosely bound electron from an isolated gaseous atom is called as first ionization energy  ${\text{(IE}}_{\text{1}}\text{).}$ Similarly, the amount of energies required to knock out second, third etc. electrons from the isolated gaseous cations are called successive ionization energies and  ${\text{IE}}_3>{\text{IE}}_{\text{2}}>{\text{IE}}_{\text{1}}.$
(i) Nuclear charge (ii) Atomic size (iii) penetration effect of the electrons (iv) shielding effect of the inner electrons and (b) electronic configurations (exactly half filled and completely filled configurations are extra stable) are the important factors which affect the ionization energies.

Similarly, the amount of energy released when a neutral isolated gaseous atom accepts an extra electron to form gaseous anion is called electron affinity.

$X\left(g\right)+e^{-}\left(g\right)\to X^{-}\left(g\right)+\text{energy}$

A positive electron affinity indicates that the ion at has a lower more negative energy than the neutral atom X. The second electron affinity for the addition of a second electron to an initially neutral atom is negative because the electron repulsion outweighs the nuclear attraction