Question

explain poor screening effect?

Answer 1

Shielding effect or screening effect is basically related to the repulsion of outermost electrons of an atom by the inner electrons .This effect is denoted by a symbol Sigma . First of all we try to understand the designation of s and p orbitals .

S as well as the P orbitals are very compact in shape and size . These two orbitals are much smaller than related d and f orbitals . So overall electron density in s and p orbitals are greater than those of d and f orbitals . Due to this compact electron density ( in other words due to highly compact electron clouds of inner orbitals ) the outermost electrons are repelled heavily by these s and p orbital electrons . This is called as strong shielding effect .

As we go down the group in Modern Periodic Table the atomic size increases ( due to increase in no. of shells , of course ! ) but Z-effective ( aka effective nuclear charge ) also increases due to involvement of d and f orbitals ( As I told you , d & f orbitals can't repel outer electrons effectively , hence increase in the size is not that much as expected ) .

Answer 2

the electrons in the atomic arbitals act as screes and block the nuclear charge passing through them .....this effect is screening or shielding effect