In the third period of the periodic table sodium is followed by magnesium aluminum silicon and phosphorus which of these elements has the smallest atomic size ?

            Phosphorous has the smallest atomic size.

Explanation:
                   As we know these elements belong to same period means there valence shell is the same. So moving from left to right along the period the shell number remains constant but the number of protons and electrons increases. So, due to increase in number of protons the nuclear charge increases hence attracts the valence electrons more effectively resulting in the decrease of atomic size.

Elements and their atomic radius are as follow,

Magnesium          0.160 nm

Aluminium           0.130 nm

Silicon                  0.118 nm

Phosphorus         0.110 nm