explain effective nuclear charge and screening effect in terms of periods and groups 

Dear Student,

Shielding effect increases with an increase in electrons. As we move left to right across a period, more electrons are being added and so shielding effect increases. But the shielding due to added electrons is small and overall results in increase in effective nuclear charge.
On moving down the group, more shells are being added and the nuclear charge also increases which decreases the size of atom, but this effect is more than offset by the screening effect of the added electrons and so the effective nuclear charge decreases and as a result, atomic radii increases.