Question

atomic size decreases across period but increases down Group of periodic table
Please who tells fast I'll mark her/him brainliest

Answer 1

The atomic size decreases across the period because effective nuclear charge on the valance shall electrons increases. In a period the the valence shell remains same for a elements of a period. 

Answer 2

atomic size decreases along period because there is increase in nuclear charge and hence electrons are held more firmly by protons and size decreases. while in a group though there is increase in nuclear charge but the effect of increase in size is more.