Which metal has the larger radius, Li or Na? Why?
Sodium has the larger radius.
The number of energy levels increases as you move down a group. The higher the energy level, the farther away it is from the atomic nucleus, and the greater the atomic radius.
Lithium is in period 2 in group 1/IA. This means that its valence electron is in the second energy level. Sodium is in period 3 in group 1/IA, so its valence electron is in the third energy level, therefore it has a greater atomic radius than lithium.
The size of an atom is usually known as its atomic radius.
The atomic radius is known as the distance from the centre of an atom to the boundary within which the electrons spend 90% of their time.
The size of an atom decreases from left to right. This is because as the number of electrons increases, the attractive forces on each electron becomes stronger. As the attractive forces become stronger, the electrons are drawn closer to the nucleus, and thus the atom is smaller.