CBSE Class 11-science Answered

In 1930 J. C. Slater devised a simple set of guidelines for shielding or screening constants ( s). For a 1s electron, s = 0.3

For electrons in an s or p orbital with n > 1, the screening constant is given by  

                         s = 1.00 N₂ + 0.85 N₁ + 0.35 N₀ 

 N₀ represents the number of other electrons in the same shell, N₁ represents the number of electrons in the next smaller shell ( n - 1 )th shell, and N₂ is the number of electrons in other smaller shells ( n - 2 and smaller shells ).

Lets calculate screening constant for an electron in the fluorine 2s orbital,

Electronic configuration of fluorine = 1s² 2s² 2p⁵

 N₀ = 6 ( the electron of interest in the L shell is not counted), N₁ = 2 (these are the 1s electrons), and N₂ = 0 (there are no other inner shells). The Slater rules predict

S = 0.85 ( 2 ) + 0.35 ( 6 ) = 3.8

n stands for principle quantum number. For 1s, n=1; 2s, n=2; 2p, n=2; 3d n=3.

In this case if we are calculating Screening constant for any electron of n shell, (n-1)th shell means other shells which are before that principal quantum number; example we are calculating for 2s orbitals electron, then (n-1)th shell is 1s;

Similarly (n-2)th shell. For example if we are calculating for 3d orbitals electron, (n-2)th shell is 1s and (n-1)th shell are 2s and 2p.