Down a group, the number of energy levels (n) increases, so there is a greater distance between the nucleus and the outermost orbital. This results in a larger atomic radius. Ionic radius is the distance from the nucleus to the outer edge of the electron cloud of an ion.

Why atomic radius increases down a group but decreases across a period?

Covalent radius increases as we move down a group because the n level (orbital size) increases. Covalent radius mostly decreases as we move left to right across a period because the effective nuclear charge experienced by the electrons increases, and the electrons are pulled in tighter to the nucleus.

Why does atomic radius increase down a group class 11?

The electrons get added in new shells which will screen the nucleus. ... So, the attraction of the nucleus for the outermost shell electron also goes on decreasing. Thus, the distance between the nucleus and the last electron will increase and hence, the atomic radius increases down a group.