Why the atomic volume of elements goes on decreasing as the atomic number increases in the third period?

atomic radius decreases as we move from left to right

The distance from the nucleus to the outermost shell depends on the electrostatic attraction (nuclear charge) that the nucleus exerts on the electrons of the outer shell. More the nuclear charge closer are the shell and electrons, hence smaller is the atomic radius of an atom.

With the increase in the atomic number (increased number of protons, electrons and neutrons) in the 3^rd period, the net positive charge of the nucleus gradually increases. This increased positive charge exerts a greater attraction on the shells and attract the electrons in the shells a little closer to the nucleus. Hence, sodium has the largest atom and chlorine the smallest