The size of an isolated atom can't be measured because we can't determine the location of the electrons that surround the nucleus. We can estimate the size of an atom, however, by assuming that the radius of an atom is half the distance between adjacent atoms in a solid. This technique is best suited to elements that are metals, which form solids composed of extended planes of atoms of that element. The results of these measurements are therefore often known as metallic radii.

The figure below shows the relationship between the metallic radii for elements in Groups IA and IIA.

There are two general trends in these data.

• The metallic radius becomes larger as we go down a column of the periodic table because the valence electrons are placed in larger orbitals.
• The metallic radius becomes smaller as we go from left to right across a row of the periodic table because the number of protons in the nucleus also increases as we go across a row of the table. The nucleus tends to hold electrons in the same shell of orbitals more tightly and the atoms become smaller.

Information provided by: http://chemed.chem.purdue.edu