Question

Why does Ar have a lower melting point than Cl2? Consequently, why do noble gases tend to have the lowest melting points in a period?

Answer 1

Ar is a noble gas and does exists as Ar atoms, hence there is no vanderwaals force present between the Ar atoms resulting in very low values of melting point and boiling point

In the period, the melting point is lowest of noble gasese because they are present as Atoms, whereas remaining all element interact with the other atom and there are forces present between other atoms leading to higher melting and boiling point as compared to the noble gases of the same period