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.