Arrange the following in the order of their increasing size
Cl-, S2-,Ca2+,Ar
At  many places i have read that we consider vanderwaal radius for noble gases and vanderwaal radius >metallic > covalent. 
If same concept is applying here, answer is not matching. 
Correct answer is S2->Cl>Ar>Ca2+
Please explain

Dear student,

Electrons in the electron cloud are constantly repelling each other and the more electrons the larger the shell. Since all of these atoms have the same number of electrons, the only thing that changes are the number of protons. The more protons the stronger the pull of the nucleus is on the electrons, so the electrons get closer. On the other side of the spectrum, S has the least number of protons meaning the weakest pull on the electrons.

The size increases as you go down the periods and towards the left of the periodic table. 
less electrons means smaller atoms. 

from smallest to largest,the correct order is S2- > Cl > Ar > Ca2+ or Ca2+ < Ar < Cl- < S2-


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