In a period as we move from left to right covalent radius decreases in a period. As we move’ from left to right atomic number or nuclear charge increases. The pull of the electron cloud by the nucleus increases, the electron cloud shrinks and as a result, the covalent radius decreases.
In a group, covalent radius increases from top to bottom. In a group, as we move from top to bottom one by one new orbital are added up thereby the size of the atom goes on increasing. The atomic radius also goes on increasing.
