Ionization Energy

The term ionization energy is applicable for all the species like atoms, molecules, radicals and ions. Generally it is used for the atoms. It is defined as: The amount of energy required to knockout the most loosely bound electron in the outermost shell in an isolated atom in the gaseous face to generate a single charge cation is referred to as the first ionization energy of the element. Similarly, the second third and higher ionization energies can be defined. For Al the involved process are shown as follows:

Al(g) → Al⁺(g) + e, IE₁ = 578 KJ mol^-1

Al⁺(g) → Al²⁺(g) + e, IE₂ = 1816 KJ mol^-1

Al²⁺(g) → Al³⁺ + e, IE₃ = 2744 KJ mol^-1

The ionization energies are expressed in eV per atom or KJ mol-1. The higher ionization energy are always greater than the lower ionization energies. It occurs so, because in removing and electron from a more positively charged species, more energy is always required this is why, IE3 > IE2 > IE1.

The ionization energy is for some molecules are given below

CO → CO⁺ + e, IE = 1352 KJ mol^-1

N₂ → N₂⁺ + e, IE = 1503 KJ mol^-1

NO → NO⁺ + e, IE = 894 KJ mol^-1

O₂ → O₂⁺ + e, IE = 1164 KJ mol^-1

The ionization energy is are very opted mistakenly described as ionization potentials because of this parameters are experimentally determined by using the potential gradient but we should remember that the potential is an intensive property measured by volt while the energy is an extensive property expressed by eV per atom or KJmol^-1.

First ionization energy trend

Periodic variation of the ionization energies

Variation trends of first ionization energy of the elements with the increase of the atomic number can be seen if we obtain a graph. The following features of the car or worth noting

a) Position of inert gases: The inert gases occupy the peak positions.

b) Position of alkali metals: The alkali metals reside at the bottom positions showing their lowest Ionization energies. From the peaks, there is a sharp fall for the respective alkali metals, then there is a gradual increase in the ionization energy with the increase of atomic number up to the climax having the position of the next inert gas.

c) Position of transition metals: In the transition series, the variation is not so mark having a slide increasing tendency, but at the endings (i.e. Zn, Cd, Hg) there are sharp hikes.

d) Position of lanthanides: In the lanthanides, again the slide increasing trained is noted having a special Hike like Gd.

Factors governing the ionization energies

The factors which affect the electrostatic attractive force experienced by the electrons at the outermost level govern the magnitude of the ionization energy. Bore theory can explain the ionization process quantitatively. The important factors to determine the ionization energy are:

a) Effective nuclear charge (Z* = Z- Screening constant)

b) Radius of the species (i.e. Principal quantum number),

c) Screening power of the inner electron clouds,

d) Penetrating power of the orbital from which the electron is to be knocked out,

e) Relative stabilities of the starting and end product species from the stand point of half-filled and full-filled character of the orbitals involved.

Variation trends in a group among the representative elements:

In the representative elements, in moving from top to bottom in a group, the size increases due to the increase of principal quantum number. On the other hand, the effective nuclear charge (Z*) for the outermost electrons increases slightly but the effect of Z* is relatively less important to the effect of n (r ∝ n² i.e. squared effect; r ∝ Z*^-1). Because of the predominant effect of the increasing principal quantum number, r (measure of size) increases in moving down in a group. This is why, the ionization energy decreases in moving from top to bottom.

For the heavier congeners, the effect of Z* is relatively more important and consequently the size (i.e. r) do not increase so remarkably (compared to the lighter congeners). This is why; the decreasing trend of ionization energy for the heavier congeners is not so pronounced (compared to the lighter congeners).

Variation trend in a period among the representative elements:

In moving from left to right in a period, the effective nuclear charge on the outermost electron increases while the corresponding principle quantum number remain the same. This is why; the size gradually decreases along the periods. Thus the electrostatic attraction towards the nucleus increases along the period and it reach the climax at inert gases. It explains the increasing trained of ionization energy along the period placing the alkali metals at the bottom and the inert gases at the peaks of the curve.

First, second, third (some fourth) ionization energies of elements in KJmol-1

Variation trade in the transition and inner transition elements:

Letters consider the comparison of variation of the first ionization energies in some group of the s-, p-, d- block elements.

It is evident that the trend of fall in ionization energy in a group towards the bottom in the representative elements is well noticed, but in the d block elements the decreasing trend is not so noticeable, rather in the heavier congeners the rivers trade appears. It arises due to both the d- and f-contractions which produce high effective nuclear charges in the heavier congeners. It arises so from the relative efficiency of the penetrating power of the orbitals which run in the sequence: s > p > d > f.

Thus the 4s and 5s orbitals penetrate into the inner d orbitals in the 4th and fifth periods respectively and the 6s orbital penetrate into the low screening 5d and 4f orbitals in the sixth period. The effect of the d- and f- contraction is also transmitted to the heavier posttransition p-block elements.

Thus, in general, for the d block elements, the trend of variation of ionization energy in a group is opposite to that observed in the representative elements. It is not an exception but rather a rule for the d block elements.

Along the series, both the d- and f- contractions and electron repulsion operate. This is why for the d block elements, only a slight increasing trend of ionization energy in a period Increase of atomic number is noticed. But when an extra stability is attend through the half-filled or full-filled structure a small height exists.

 Reference

1) Concise inorganic chemistry by J. D. Lee.

2) Inorganic Chemistry by James E. Huheey, Ellen A Keither, Richard L. Keither, Okhil K. Medhi.

3) Shriver and Atkins Inorganic Chemistry.