Ionization Energy.

➨Ionization Energy (IE) is defined as the amount of energy required to remove the most loosely bound electron from an isolated gaseous atom to form a cation. 

M(g) + IE1 ➞ M+ + e-

M+(g) + IE2 ➞ M2+ + e-

M2+(g) + IE3 ➞ M3+ + e-

➨IE1, IE2 and IE3 are the 1st, 2nd and 3rd ionization energies to remove electron from a neutral atom, monovalent and divalent cations respectively. 

➨In general, IE1<IE2<IE3<.......... because as the number of electrons decreases, the attraction between the nucleus and the remaining electrons increases considerably and hence subsequent ionization energy increase. 

➨Ionization energies are determined from spectra and are measured in KJ/mol, KCal/mol.

➨Variations in ionization energies in a period and group may or may not be regular and can be influenced by the following factors:-

➥Size of the atom:- Ionization energy decrease with increase in atomic size. As the distance between the outermost electrons and the nucleus increases, the force of attraction between the valence shell electrons and the nucleus decreases. As a result, outermost electrons are held less firmly and lesser amount of energy is required to knock them out. 
For example, IE decreases continuously in a group from top to bottom with increase in atomic size. But in 13th and 14th groups the ionization energy doesn't decrease continuously and order is as follows:-

For 13th group:- B > Tl > Ga > Al > In

 For 14th group:- C > Si > Ge > Pb > Sn

➥Nuclear Charge:- The ionization energy  increases with increase in the nuclear charge. This is due to the fact that with increase in the nuclear charge, the electrons of the outermost shell are more firmly held by the nucleus and thus greater amount of energy is required to pull out an electron from the atom. 

For example, IE increases as we move from left to right along a period due to increase in nuclear charge. 

➥Shielding or Screening effect:- The electrons in the inner shells act as a screen or shield between the nucleus and the electrons in the outermost shell. This is called shielding effect. The larger the number of electrons in the inner shells, greater is the screening effect and smaller the force of attraction and thus ionization energy decreases.

➥Penetration effect of the electron:- The IE also depends on the type of electron which is removed. s, p, d and f electrons have orbitals with different shapes. An s-electron penetrates more closer to the nucleus, and is, therefore, more tightly held than a p-electron. Similarly, p-orbital electron is more tightly held than a d-orbital electron and a d-orbital electron is more tightly held than an f-orbital electron. If other factors being equal, IE are in the following order:-

s > p > d > f

For example, IE of Al is comparatively less than Mg because outermost electron is to be removed from 3p-orbital in Al whereas in Mg it will be removed from 3s-orbital of same energy level. 

➥Electronic Configuration:- If an atom has exactly half filled or completely filled orbitals, then such an arrangement ha extra stability. The removal of an electron from such an atom requires more energy than expected. 

For example, IE1 of Be is greater than B because Be has extra stable completely filled outermost 2s-orbital while B has partially filled less stable outermost 2p-orbital.

Be (Z=4) ➞ 1s2, 2s2

B (Z=5) ➞ 1s2, 2s2, 2p1

Similarly, noble gases have completely filled electronic configuration and hence the have highest ionization energies in their respective periods. 

➨Metallic or electropositive character of elements increases as the value of IE decreases. 

➨The relative reactivity of the metals in gaseous phase increases with the decrease in IE. 

➨The reducing power of elements in gaseous phase increases as the value of IE decreases. Amongst alkali metals, the Li is strongest reducing agent in aqueous solution. 

End

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