Q.What is screening effect?

Inner-shell electrons partly cancel the nucleus's pull on a valence electron by repelling it -- this is the shielding (screening) effect -- so the net attractive charge a valence electron actually feels, the effective nuclear charge, is less than the full nuclear charge:

where $S$, the screening constant, is estimated by Slater's rules:

1. Group the electron configuration as $(1s)(2s,2p)(3s,3p)(3d)(4s,4p)(4d)(4f)(5s,5p)\ldots$
2. Electrons in a group to the right of the electron of interest contribute nothing; electrons in the same group contribute 0.35 each (0.30 if the electron of interest is 1s).
3. For an s or p electron of interest: electrons one group inward $(n-1)$ contribute 0.85 each; electrons two or more groups inward contribute the full 1.00 each. For a d or f electron of interest: every electron to its left contributes the full 1.00, with no intermediate 0.85 tier.
4. Sum all contributions to get $S$. …