J.C.Slater prposed an empirical constant that represents the extent to wwhich the other electrons of the atom shield ( or screen ) any particular electron from the nuclear charge . thus slaters screening constant is used in the following equation :

        Z*  =  Z - sigma

here , Z is the atomic number of the atom , and hence is equal to the actual number of the protons in the atom . the parameter Z* is the effctive nuclear charge , which according to the above equation is smaller that Z , since the electron n the question is screened ( shielded ) frm Z by an amount sigma . we found that for cases in which the screening is small , the effective nuclear charge is large . conversely , an electron that is well shielded ( large value for sigma in the above equation ) from the nuclear charge Z experiences  small effective nuclear charge Z* .

the value for sigma for any one electron in the given eectronic configuration ( i.e  in the presence of other electrons of the atom in the question ) is calculated using a set of empirical rules developed by slater . according to these rules , the value of sigma for the electron in question is the cumulative total provided by the various other electrons of the atom . the other electrns of the atom add an intrinsically different contribution to the value of s as follows :

if the electron in the question resides in a s or a p orbital

1. all electrons in the principle shell higher than the electron n the question contribute zero to sigma .

2. each electron ion the same shell contributed 0.35 to sigma

3. electrons in the (n-1) shell each contribute 0.85 to sigma .

4. electrons n the deeper shel contribute 1 to sigma .

if the electron in the queston resides in a d or f orbital

1. all electrons in principle shell higher than theelectron in question contribute zero to sigma

2. each electrn in the same principle shell contribute 0.35 to sigma .

3. all inner shell electrons ( i.e. (n-1)  and lower ) uniformly contribute 1 to sigma .