Question

Among alkali metals __ has the smallest atomic radius.

Answer 1

The radius of an atom is governed by two factors:

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

Compare the electronic configurations of lithium and sodium:

• Li: 1s²2s¹
• Na: 1s²2s²2p⁶3s¹

In each element, the outer electron experiences a net charge of +1 from the nucleus. The positive charge on the nucleus is canceled out by the negative charges of the inner electrons. This is true for each of the other atoms in Group 1. The only factor affecting the size of the atom is the number of layers of inner electrons which surround the atom. More layers of electrons take up more space, due to electron-electron repulsion. Therefore, the atoms increase in size down the group.

Hence among alkali metals, Lithium has the smallest atomic radius.