Question

why the size of Ar is greater than Cl give reason​

Answer 1

Answer:

As we move from left to right in in a periodic table, the atomic radius decreases. This is because (while moving from left to right) no of protons increases which results Protons pulling electrons closer to the nucleus.

Now this behaviour breaks when we reach at Noble elements (such as Ne, Ar) where the size of atoms abruptly increases.

This is because in Noble elements the last shell (and therefore all its orbitals) are completely filled. Now due to this fullness the electrons from the lower energy shells are not able to excite to the outermost higher energy shell, thus generating a repulsion between energy levels, which in turn results in increament of size of atom.