又係chem問題

Monoatomic means the basic unit consists of one single atom only. It can be used to describe molecules or ions.And it's an ion that formed from one element(or one kind of atoms).

All of the noble gases are monatomic because they are very unreactive and are not combine with others(e.g. oxygen,hygrogen).Therefore, it's monoatomic.

Hope These Information Can Help You To Understand Chemistry More~ =]