[sabbiikram]

what is sheilding effect?
How it is apply to electronic configuration i mean somtimes we write outer orbital(e.g.5s) first than inner orbital(e.g.4d)?

[renge ishyo]

The shielding effect occurs when an electron is placed in an outer orbital surrounding a nucleus that already had inner electrons surrounding the nucleus previously. The inner electrons "shield" the nucleus by preventing the additional electron from feeling the full positive charge of the nucleus (i.e. the inner electrons repel the newly added electron away from the nucleus which counteracts the pull on the added electron towards the nucleus due to the positive charge of the nucleus).