1.6 Periodic Trends

Atomic Radius

The atomic radius is the size of an atom, or the distance between the nucleus and the valence shell. As you go down the group (column), the number of energy levels increases, so the atomic radius increases. Sodium is lower than lithium, so sodium would have a higher atomic radius.

Atomic radius decreases from left to right. As you go across a period (left to right), the amount of protons in the nucleus increases. This increases the effective nuclear charge (${Z_eff}$), which is the net positive charge of the atom’s nucleus on its electrons. ${Z_eff}$ is the number of protons minus the number of core electrons; the protons attract the electrons to the nucleus while the core electrons repel the valence electrons. Atoms with higher ${Z_eff}$ have a lower atomic radius because the valence electrons are closer to the nucleus. Beryllium is to the right of lithium, so it has a smaller atomic radius.