Ionisation energy

About Ionisation energy

In an atom, the electrons are held by the nucleus. To remove an electron, the nuclear pull has to be overcome. To overcome the nuclear pull, the electron needs energy. The electron, after absorbing energy, overcomes the nuclear pull and becomes free from the atom, leaving behind positive ion. Thus, ionisation takes place and the energy absorbed by the electron is called Ionisation energy’.
Ionisation energy can be defined as “the amount of energy required to remove the outermost electron from an isolated gaseous atom”.

For example: Sodium exists in a solid form. When heated, it forms vapours constituting gaseous isolated and the sodium atoms. To remove an electron from this gaseous isolated atom, energy equivalent to the ionisation energy is needed. The process can be represented as:
Na(g) + Ionisation energy Na(g)⁺ + e^–

Units: The ionisation of energy is measured in mole KJ/mole (or) Kcal/mol (or) eV/atom
1eV/atom = 23.06 Kcal/mole
1 eV/atom = 96.45 KJ/mole
1 KJ/mole = 4.126 Kcal/mole

Significance of ionisation energy

 Ionisation energy gives us an idea of how tightly an outer electron can be held by’ the atom. The more the Ionisation energy, the more tightly is the outermost electron held by the atom. The Ionisation energy is an indication of how easily an atom of an element forms cation
A + I.P.  A+ + e^–

The more the Ionisation energy, the more difficult it is to form a cation.

Successive Ionisation Energies:

The 1^st ionisation results in a unipositive ion. The equation can be written as:

M(g) + IE₁ M+(g) + e^–

When another electron is removed from this unipositive gaseous ion, the energy required to do so is called the second ionisation energy (I.E₂). 

M+ + IE₂  M⁺⁺ + e^– 

One must think that whether the first ionisation energy (IE1) and the second ionisation energy (I.E₂) be similar? 

To explain this question lets discuss The first ionisation results in a unipositive ion. During the second ionization, the electron has to be removed from the unipositive ion. The positive charge of unipositive ion results in an extra nuclear pull and more energy is needed to remove an electron.
Hence, second ionisation energy is greater than the first ionisation energy. Similarly IE₃> IE₂..... IE_n > IE_n–1

Relationship between Ionisation energy and atomic size

To know the variation of Ionisation energy with respect to size, let us study the variation of  Ionisation energy in a group and a period. As we move down the group the atomic size increases and the nuclear pull decreases. The weak nuclear pull results in the outer electron ting held loosely, thereby requiring less energy to remove the outer electron and hence, Ionisation energy decreases. Thus, down the  group the Ionisation energy decreases due to decrease in size.

Ionisation energy and electronic configuration

The smallest element among the second period elements is Fluorine and we expect the Ionisation energy to be maximum. But surprisingly, the ionsation potential is more for the next element Neon, in spite of its being the biggest element of the 2_nd period.
Let us observe the electronic configuration of Fluorine and Neon.
The electronic configuration of ‘F’ is 1s², 2s², 2p⁵ and that of ‘Ne’ is 1s², 2s², 2p⁶.
The number of electrons in the outer shell of Ne are eight and it has a stable octet configuration. It is difficult to remove an electron from such a stable configuration. A large amount of energy is needed for this to happen. Thus Ne, in spite of its bigger size, has greater Ionisation energy than fluorine due to its stable configuration. Check out Chemistry Formulas and NCERT Solutions for class 11 Chemistry prepared by Physics Wallah.