Trends in atomic radius of Group 2

The atomic radius increases down the group. Notice that beryllium has a particularly small radius compared with the rest of the group.

The radius of an atom is governed by

1. the number of layers of electrons around the nucleus
2. the attraction the outer electrons feel from the nucleus.

Compare the electron configurations of beryllium and magnesium:

• Be: 1s²2s²
• Mg: 1s²2s²2p⁶3s²
   

In each case, the two outer electrons experience a net charge of +2 from the nucleus. The positive charge on the nucleus is balanced by the negativeness of the inner electrons.

This is true for all the atoms in Group 2.  The only factor influencing the size of the atom is therefore the number of layers of inner electrons that fit around the atom. More layers of electrons take up more space, due to electron-electron repulsion, so atoms are larger down the group.