Question

Why does atomic radii decrease across a period and increase down a group?

Answer 1

The atomic radius decreases across the periodic table for a few reasons:

1.  Increase in effective nuclear charge. The effective nuclear charge is the net positive charge experienced by an electron in a polyelectronic atom i.e. an atom containing more than one electron. Basically, the number of protons in the nucleus increases from left to right across a period. This means there is a greater attractive force on the outer electrons. The energy levels draw closer to the nucleus because of this; causing the atomic radius to decrease.

2.  No increase in the screening effect. The screening effect: the attractive force of the inner nucleus is lessened the more electrons and orbits that are present i.e. the inner orbits shield the outer electrons from the positive charge of the nucleus. When you move from left to right across a period, the extra electron is being added into the same outer energy level. This, again, causes the atomic radius to decrease. (If the screening effect were to be a factor then the atomic radius would increase, which is part of the reason that the values of atomic radius increase down the groups).

So, in short, the increase in effective nuclear charge causes the energy levels to draw closer to the nucleus and the fact that there is no increase in the screening effect ensures that the atomic radius decreases across a period.

Answer 2

Atomic radius decreases across a period because shells remain same but no of valence electrons increases so force of attraction between nucleus and valence electrons increases.

Atomic radius increases down a group as valence electrons remain same but no of shells increases and force of attraction between nucleus and valence shell decreases.