It is possible to evaluate shielding or screening constant (s) which is defined as a measure of the extent to which the intervening electrons shield the outer electrons from the nuclear charge. SI. gave a useful set of empirical rules for calculating the effective nuclear charge (Z ) experienced by electrons in different orbital’s . As stated above, the Z_eff acting on a given electron is calculated by subtracting the screening constant (S) from the atomic number  Z (nuclear change)That is,
                                                        

The “Slater rules” for calculating screening or shielding constant (S) are as follows :

1.The electron configuration of the element is first writing the following order and grouping :
 
(1s); (2s,2p); (3s,3p); (3d); (4s,4p); (4d); (4f); (5s,5p); (5d); (5f): and so on.
2. For an electron considered in a group of s, p-electrons, the shielding constant (s) is the sum of the following contributions:
(a) Nothing for electrons in groups beyond (outside) the one considered.
(b) An amount of 0.35 for each other electron in the group considered (except in the 1s group where 0.30 is used instead of 0.35).
(c) An amount of 0.85 for each electron in the next inner shell (n-1).
(d) An amount of 1.00 for each of all other inner electrons.

3. For an electron considered in d or f group, rules 2a and 2b apply as such. However, rules 2c and 2d are replaced with the rule that all other electrons lying to the let of d or f group contribute 1.00 each.

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