Diatomic and Monoatomic

Why do we represent the molecule of solid metals using the symbol of the element itself(for example ,molecule of iron=$\ce{Fe}$,etc.) whereas we represent the molecule of the elements like oxygen in a different way(we represent molecule of oxygen as $\ce{O2}$,not $\ce{O}$)?

I think that we do so because the metals can exist in atmosphere as single atoms whereas nonmetals do not.

Am I correct?

If I am correct ,then why is it such that metals can exist as single atoms whereas nonmetals need to form bonds in order to exist in the atmosphere?

When we refer to a metal, we refer to the bulk solid, not to a molecule. How many iron atoms are there in a block of iron? If metals do form molecules, we will represent them as such, for example with dilithium, $\ce{Li2}$ (no, not the Star Trek variety).