screening effect and Slater's rule

Screening effect (or shielding effect):

The electron of valence shell (outer most shell)  are attracted towards the nucleus also the valence electron are repelled by the electron present in time shell.

This decrease in force of attraction exerted by the nucleus ,on valence electron due to presence of electron in inner shell, is called screening effect or shielding effect.

More are the no.  of electron in the inner shell more more will be screening effect. 

slater's  rules:  Slater gave a set of emperical rules for calculating effective nuclear charge z(eff)  z* effective nuclear charge acting on a given electron is caluculated by substracting the screening constant from atomic number
 (z)   

z(eff)  = atomic nio.  -  screening constant
  
z(eff)  = Z- S

Rules 
1.are the purpose of estimitating screening constant ns and np electron considered together in a single group while nd and nf electron each from separate group. 

2.write down electronic configuration of element in following order and group. 

1s  2s 2p   3s 3p 3d     4s 4p 4d 4f  

3.contributing made by each electron present in same shell = 0.35

4.contributing made by each electron present in pentivalent shell 0.85

5.contributing made by each electron present in further inside =1.00

6. all electron in a group lying to the left of nd and nf group contribute =1.00