Define `IE_(1)` and `IE_(2)`. Why is `IE_(2) > IE_(1)` for a given atom? Discuss the factors than effect IE of an element.

Explanation

The minimum amount of energy required to remove an electron present in the outermost orbit of a neutral, isolated gaseous atom is called ionisation potential. It is denoted by `I_(1)` and is measured in kilocalories or electron volts.
`M(g)+I_(1) rarr M_(g)^(+) + e^(-)`
The amount of energy required to remove another electron from a uni+ve ion is called second ionisation potential. It is denoted as `I_(2)`.
`M_(g)^(+) + I_(2) rarr M_(g)^(+2) + e^(-)`
Ionisation energy is measured in ev`//`atom (or) k cal`//`mole (or) KJ`//`mole.
The uni +ve ion formed, by the removal of an electron from a neutral atom will have more nuclear attractions over the electron cloud because the number of electrons. As a result of which more energy is required to remove and electron form this uni +ve ion than a neutral atom. Hence second I.P values are always greater than first I.P vaues.
Factors affecting Ionisation potential :
1. Atomic radius : As the atomic radius increases, the distance between the nucleus and the outermost electons increases. Hence, the effective nuclear charge on the outermost electorns decreases. As a result of which the energy reuqired to remove the electrons decreases. From this it is evident that as atomic radius increases I.P decreases. Similarly as atomic radius decreases I.P increases.
(2) Nuclear Charge: As nuclear charge increase, the nuclear attractions over the valence electrons increases. So, more amout of energy is required to remove these electrons. From this it is evident that as nuclear charges increases I.P. increases. Similarly as nuclear charge decreases I.P. decreases.
(3) Screening effect (or) Shieilding effect: In multi-electron present in the shells screen the electron present in the outermost shells from the nuclear attractions. This effect dependes upon the number of screens (no. of inner orbitals). As the screenig effect increases, ionisation pontential decreases. Similarly as the screening effects decreases, I.P. increases.
(4) Extent of penetration of orbitals of valence electrons: As the penetration power of orbitals increases, IP also increases. Order of penetration of orbitals is S > P > d > f. Hence, order of IP is also S > P > d > f.
(5) Electron configuration: Atoms of elements with half-filled or completely filled electron configuration are more stable. Such atoms have more IP values.