Ionization energy basics and advanced concepts

Basics and advanced concepts of Ionization energy at: http://www.adichemistry.com/general/periodictable/ip/ionization-energy.html

Energy is required to overcome the attraction between nucleus and the electron while removing it from the atom. This energy is referred to as ionization energy and is expressed in kJ mol^-1. It is a measure of nuclear attraction over the outermost electron in a given species (atom or ion or molecule).

Previously it was denoted by ionization potential expressed in eV atom^-1. Physicists still follow this convention.

The ionization of an atom or an ion or a molecule (by removing electron) results in a positively charged species and is always an endothermic reaction i.e. energy is absorbed. Therefore, the enthalpy of ionization is a positive quantity.

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FACTORS AFFECTING IONIZATION ENERGY

1) Atomic radius:

It is easy to recognize that the force of attraction between nucleus and electron decreases with increase in the distance between them. As a consequence, the  ionization energy decreases with increase in atomic radius since the nuclear attraction over outer electron decreases. Thus farther the electron from nucleus, easier will be the removal.

E.g., Down the group, the ionization energies decrease with increase in atomic radii.

Order of ionization energies in IA group ——->  Li > Na > K > Rb > Cs > Fr

2) Effective nuclear charge:

Ionization energy increases with increase in the effective nuclear charge. It is evident from the trend observed in first and second ionization potentials.

Effective nuclear charge refers to the nuclear attraction experienced by the electron. It is proportional to the number of protons per electron (i.e. ratio of protons to electrons).

 

3) Shielding or screening effect:

Reduction in nuclear attraction over the outer electrons due to repulsions by inner electrons is called screening or shielding effect.

Ionization energy decreases with increase in screening power of inner electrons.

The order of screening power of electrons present in different types of orbitals is: s > p > d > f .

4) Penetration power:

The ability of an electron to come closer towards nucleus is called penetration power. Electrons will have the natural tendency to come closer towards the nucleus. However, the time they spend closer to the nucleus depends on the shape of orbital as well as the principal quantum number.

Greater the penetration power, greater will be the ionization energy.

The order of penetration power of electrons in different orbitals is as follows.

s > p > d > f

5) Electronic Configuration:

Atoms with stable electronic configurations like: ns² np⁶ or half filled or completely filled sub shells are extra stable. Removal of electrons from them is difficult and hence their ionization energies are comparatively high.

These configurations are more symmetric and the exchange energy is minimal. Hence they are relatively more stable.

E.g. Noble gases with ns² np⁶ ; IIA group elements with ns² (completely filled); and VA group elements with ns² np³ (half filled) configurations have higher ionization potentials.

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