All about radius of atoms: determination

The radius of an atom can be estimated by taking half the distance between the nucleus of two of the same atoms.

Going across the periodic table, the atomic radius decrease: This is due to the fact that the principle energy level (principle quantum number) remains the same, but the number of electrons increase. The increase in the number of electrons cause an increase in the electrostatics attraction which cause the radius to decrease.

Going down the periodic table, the atomic radius increase: The principle energy level increases and hence the atomic radius increases.