Question

What is screening effect? How is it related to I.E?

Answer 1

Force felt by the valence shell electrons from the electron present in the inner shell is called the shielding effect or screening effect.
Ionization energy: The minimum amount of energy which is required to remove an electron from the valence shell or outer most shell is called ionization energy.

Complete answer:
The actual charge felt by the valence shell electron is called effective nuclear energy. It is denoted by ${{\text{z}}_{\text{eff}}}$ and the repulsive forces felt by valence shell electrons from the electron present in the inner shell is called shielding effect or screening effect.
${{\text{z}}_{\text{eff}}}$= total nuclear charge$-$ screening constant ($\sigma$)
Screening constant = total nuclear charge$-{{\text{z}}_{\text{eff}}}$
Greater the number of electrons in inner shells larger will be the screening effect. With increase in screening effect ${{\text{z}}_{\text{eff}}}$ will be decreased.
As the force of attraction by the nucleus for the valence shell electrons decreases and hence the ionization energy decreases. In other words an increase in the number of electrons in the inner shell leads to decrease the ionization energy.

Note:
-The value of $\sigma$ (screening constant) is calculated by the slater rules
Slater rules: It is defined as the actual charge experiment by an electron is equal to the charge to be formed by the number of protons but minus a certain value of charge from other electrons.
-Write the electronic configuration in the following way
(1s) (2s, 2p) (3s, 3p) (3d) (4s, 4p) (4d) (4f).
-Any electrons to the right of the electrons have no contribution in shielding.
-All other electrons in the same group contribute $0\cdot 35$ nuclear charge.
-Electrons having one valence less than principal quantum number shield to an extent of $0\cdot 85$ units of nuclear charge.
-Electrons having less than two value from principal quantum number contribute $1\cdot 00$ units
-The electrons in d e f orbitals contribute$1\cdot 00$unit of nuclear charge.
-Taking the sum of all the units then we calculate the screening effect and effective nuclear charge.