The radii of the atoms become smaller from sodium to chlorine across period 3. This decrease is primarily a result of

Nuclear Charge

Explanation:

As we move from sodium to chlorine across, there's increase in atomic number (protons). This also means there is increase in the valence electrons (The valence electrons is denoted by the group).

As the number of protons increases, the nuclear energy also increases. Due to this, the electrons are attracted more strongly and pulled closely to the nucleus.

increased stability in the completion of fluorine's outer shell.