The radius of an atom is defined as half the distance between the nuclei of two atoms in a pure sample of the substance. The measurements have been obtained through x- ray diffraction techniques. The periodic table displays two significant relationships about the radii of atoms: 1) as elements increase in atomic number in a period, going from Group IA to Group 7A, the atomic radius decreases; 2) as elements in the same group increase in atomic number, the atomic radius increases. The largest atom has a radius that is only twice the size of the smallest.

The reason that the radii of the atoms within a period decreases relates to the electric force between the positively charged nucleus and the negatively charged electrons orbiting it. The number of protons in the nucleus is increased from Group IA to Group 7A. Adding protons to the nucleus results in a greater electric force. When the force of attraction increases, the electrons are drawn closer to the nucleus.

The reason that the radius of the atom increases within a group as the period number increases is the location of outer electrons in higher energy levels. These electrons have a higher probability of being located farther from the nucleus, resulting in a larger atomic radius. There is also a shielding effect by the core electrons, decreasing the force of attraction between the nucleus and the valence electrons.