Question

explain shielding effect across d period and along d group .

Answer 1

As we know that electron fill in multiple shells in an atom according to energy. The size of an atom depends on the attractive force between nucleus of an atom and outermost electrons. The electrons occupied in inner orbitals works as shield in between the attractive forces of the nucleus and the outermost electron. So as the number of electron in inner orbitals increases it cause more shielding and hence lesser attractive force which results in bigger size. So in short we can say that shielding effect reduces the attractive forces for outermost electron. s orbital has the best shielding effect followed by p obital, d orbital and the poorest shielding effect is of f orbital .
In case of d-block element as we move from left to right the number of electron increases in inner d-orbital so it should cause shielding and increase in size but it is observed that covalent radii decreases from left to right in series until near the end when the size increases slightly. This is because of poor shielding of d-electrons. So we can say that shielding increases from left to right that is why size is increased at the end of the series.
As we move from top to bottom in d-block , size increases down the group but increase is lower as compared to s and p block elements. This shows that shielding effect decreases as we move down the group that is why increase in size is lower from expected.