What Is Ionization Energy

IONISATION ENERGY (IE)

Ionisation Energy(IE) of an element is defined as the amount of energy required to remove an electron from an isolated gaseous atom of that element resulting in the formation of a positive ion.

The energy required to remove the outermost electron form an atom is called first ionization energy (IE)₁. After removal of one electron, the atom changes into monovalent positive ion.

M(g) + IE₁ → → M+(g) + e-

The minimum amount of energy required to remove an electron from monovelent positive ion of the element is known as second ionization energy (IE)2.

M+(g) + IE₂ → → M2+(g) + e-

The first, second etc. ionization energies are collectively known as successive ionization energies. M2+(g) + IE₂ → → M3+(g) + e-

In general (IE)₁< < (IE)₂ < < (IE)₃ so on, because, as the number of electrons decrease, the attraction between the nucleus and the remaining electrons increases considerably and hence subsequent ionization energies increase.

FACTORS INFLUENCING IONIZATION ENERGY:

Size of the atom:

Ionisation energy decreases with increase in atom 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, ionization energy decreases in group from top to bottom with increase in atomic size.

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 form the atom. For example, ionization energy increases as we move from left to right along a period due to increase in nuclear charge.

Shielding 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 or screening effect. Large 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 electrons:

The ionization energy increases as the penetration effect of the electrons increases. It is a well known fact that the electrons of the s-orbital have the maximum probability of being found near the nucleus and this probability goes on decreasing in case of p,d and f orbitals of the same energy level. Greater the penetration effect of electrons more firmly the electrons will be held by the nucleus and thus higher will be the ionization energy of the atom.

For example, ionisaion energy of aluminium is comparatively less than magnesium as outermost electron is to be removed from p-orbital (having less penetration effect)( in aluminium, whereas in magnesium it will be removed from s-orbital (having larger penetration effect) of the same energy level.

Electronic Configuration

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

IE1 of Be > > IE₁ of B

As noble gases have completely filled electronic configurations, they have highest ionization energies in their respective periods.

VARIATION OF IONIZATION ENERGY IN A PERIOD:

In general, the value of ionization energy increases with increases in atomic number across a period. This can be explained on the basis of the fact that on moving across the period from left to right-

• Nuclear charge
• Addition of electrons occurs in the same shell
• Atomic size decreases

VARIATION OF IONIZATION ENERGY IN A GROUP:

In general, the value of ionization energy decreases while moving from top to bottom in a group. This is because –

• Effective nuclear charge decreases regularly
• Addition of electrons occurs in a new shell
• Atomic size increases

Conclusions :

• In each period, alkali metals show lowest first ionization enthalpy. Caesium has the minimum value of IE
• In each period, noble gases show highest first ionization enthalpy. Helium has the maximum value of first ionization enthalpy
• The representative elements show a large range of values of first ionization enthalpies, metals having low while non-metals have high values.
• Generally, ionization enthalpies of transformation increase slowly as we move from left to right in a period. The f-block elements also show only a small variation in the values of first ionization enthalpies.

Example-1

Second ionization enthalpy (IE2) for alkali metals (Li, Na, K etc.) is very high as compared to their IE1 values. Explain why?

Solution: This is due to following reasons:

(i) After the removal of first electron, the atom changes into a positive ion in which the electrons are held more tightly by the nucleus.

(ii) The second electron is to be removed from a lower energy level and more energy is required for this purpose.

(iii)Positive ion formed by the removal of first electron in case of alkali metals has a stable noble gas configuration.

Due to the similar reasons, the third I.E. values for Be and Mg are also very high.

Example -2

The first and second I.E. of Mg are 750 and 1450 kJ/mole respectively. What will be the ratio of Mg+ and Mg2+ ions, if one mole magnesium vapours are supplied 1200 kJ of energy?

Solution: Mg(g) → Mg+ (g) + e– IE₁ = 750 kJ mol–1 …(i) (given)

Mg+(g) → Mg2+ (g) + e– IE₂ = 1450 kJ mol–1 …(ii) (given)

Amount of energy needed for the conversion of one mole Mg (g)  Mg+ (g) = 750 kJ, therefore, amount of energy left for the conversion of,

Mg+(g) →Mg2+(g) + e–, i.e., 1200 – 750 = 450 kJ.

From eq. (ii), 1450 kJ energy converts, Mg+ (g) → Mg2+ (g) = 1 mole

450 kJ energy converts Mg+ (g) → Mg2+ (g) = 450/450mol = 0.31 mole.

Thus, Mg2+ (g) = 0.31 moles and Mg+ (g) = 1 – 0.31 = 0.69 moles.

Therefore, Mg+ (g) : Mg2+ (g) = 0.69 : 0.31.