The energy required to remove an electron from the outer most orbit
or shell of a neutral gaseous atom is called ionization energy. The energy
required to remove the first electron from the outer most orbit or shell of
a neutral gaseous atom of the element is called its first ionization energy.
The energy required to remove an electron from uni- positive ion of the
element is called the 2nd ionization energy of that element and so on.
1. Nuclear charge: more the nuclear charge more is the ionization
energy.eg:Between 11 Na and17 Cl, Chlorine atom has more ionization energy.
2. Screening effect or sheilding effect: more the shells with
electrons between the nucleus and the valence shell, they act as screens
and decrease nuclear attraction over valence electron. This is called the
screening effect. More the screening effect, less is the ionization energy.
Between 3Li and 55 Cs, the element 55 Cs with more inner shells has less
ionization energy.
3.Penetration power of the orbitals: orbitals belonging to the same
main shell have diffrent piercing power towards the nucleus, for examle
4s>4p>4d>4f in the penetration. There fore, it is easier to remove 4f
electron than 4s. Between 4
Be 1s2 2s2
and 5B 1s2 2s22p1, the element B has
less ionization energy due to less penetration power of ‘2p’ compared to
‘2s’
4. Stable configuration: it is easier to remove one electron from 8O
(1s22s22p4) than 7
N (1s22s22p3). This is because, 7N has stable half filled configuration.
5. Atomic radius: more the atomic radius, less is the ionization energy.