Molecules of many elements, such as argon (Ar), helium (He), neon (Ne), etc., are made-up of only one atom of the element because:

i. These elements are noble gases which are unreactive in nature.

ii. They have a completely filled outermost shells.

iii. As a result, they are unable to combine with any other element.

iv. Hence, they exist independently in the form of a single atom.