Why is argon bigger than chlorine inspite of the fact that atomic radius decreases from left to right across a period?

Explanation

Chlorine atoms form diatomic molecules. The two chlorine atoms in a chlorine molecule are held together by strong covalent bond. Therefore, its covalent radius which is one half of the distance between nuclei of two chlorine atoms is small on the other hand argon is monoatomic. The atoms are held together by weak forces called van der Waals. forces. So, its radius is one half of the internuclear distance between two close argon atoms (not bonded) hence, its atomic radius in large.