CBSE Class 11-science Answered

Ionization energy depend on the following factors:

Size of the atom

As the size of the atom increases the outermost electrons are held less tightly by the nucleus (attractive force between the electron and the nucleus is inversely proportional to the distance). As a result it becomes easier to remove the electron and therefore the ionization energy decreases with the increase in atomic size.  

Charge on the nucleus

The attractive force between the nucleus and the electron increases with the increase in nuclear charge making it more difficult to remove an electron. The ionization energy thus increases with the increase in the nuclear charge.

Screening effect in multielectron atoms

The outermost electrons are shielded or screened from the nucleus by the inner electrons. This is known as shielding or screening effect. The outer most electrons do not feel the complete charge of the nucleus and the actual charge felt is called the effective nuclear charge. When the inner electrons are more, the screening effect will be large, the nuclear attraction will be less. Thus when the inner electrons increase the ionization energy will decrease.  

Penetration effect

The 's' electrons are more penetrating (maximum probability of finding near the nucleus) towards the nucleus than the p electrons. The order of penetration power in a given shell is s > p > d > f . If the penetration power of the electron is more, it will be closer to the nucleus and will be held more firmly. Thus ionization energy will increase with the increase in the penetration power of the electrons. For the same sub shell the ionization energy would be more to remove an 's' electron than to remove a 'p' electron which in turn will be more than that for removing a 'd' electron.  

Electronic arrangement

Certain electronic configuration like half-filled and completely-filled shells have extra stability. It is more difficult to remove electron from these stable configuration and the ionization energy is very high. For example, thenoble gases have the most stable configuration and so have high ionization energy; elements like Be and Mg have completely filled orbitals while N and P have exactly half-filled sub shells. Thus, their ionization energies are high. The more stable the electronic configuration, the higher is the ionization energy.