Question

9. Explain why the atomic radii decrease from left to right through a period of elements (in the periodic table), but increases from top to bottom through a group of elements?

Answer 1

While moving from left to right in a period the number of shells remains same but effective nuclear charge , Z_eff keeps on increasing which attracts/pull the outer electrons towards itself more tightly and thus leads to decrease in the atomic radii.

Where as, while moving down a group ie from top to bottom, apart from increasing the nuclear charge Z_eff , now the number of shells also increases thus the valence electrons are now too far from the nucleus to experience less effective nuclear charge Z_{eff ,} which results in increase of atomic radii