Question

The shielding constant $\sigma $ for $_{7}N$ atom is:
A. 3.10
B. 3.40
C. 3.80
D. 4.20

Answer 1

Hint: The shielding constant is calculated with formula given by slater’s rule and the general principle behind this rule is to describe the actual charge felt by an electron is equal to the charge of certain number of protons present in them and but minus a certain amount of charge from other electrons.

Complete Solution :
Shielding constant is calculated by slater’s rule which have the formula
${{Z}^{*}}=Z-\sigma $
Where ${{Z}^{*}}$ is effective nuclear charge, Z = atomic number and $\sigma $= shielding or screening constant.
- The shielding constant for each group is formed as the sum of the following contributions:
- Electrons in the same group as the electron of interest shield to an extent of 0.35 nuclear charge units except 1s group in which the other electron contributes only 0.30.
- If the group is of s,p type then an amount of 0.85 from each electron in (n-1)th shell and an amount of 1.00 for each electron from (n-2) and lower shells is added to the shielding constant.
- If the group is of [d] or [f] type, an amount of 1.00 for each electron from all lying left to that orbital.
- Now in the given question electronic configuration is $1{{s}^{2}}2{{s}^{2}}2{{p}^{3}}$
Screening constant $\sigma $ = $(0.35\times 4)+(0.85\times 2)=3.10$
So, the correct answer is “Option A”.

Note: In an atom electrons present in the valence shell are attracted to the nucleus and also these electrons are repelled by the electrons present in the inner shells. Due to this the actual force of attraction between the nucleus and the valence electrons is decreased by the repulsive forces acting in opposite directions. This decrease in the force of attraction exerted by the nucleus on the valence electrons is due to the presence of electrons in the inner shells and this effect is called screening effect or shielding effect.