I was thinking of a substance I could use to describe particle arrangement in solid/liquid/gas phases to school kids, and after realising water would be bad to use (since the liquid is denser than solid), I thought of using metal elements since they can exist in all phases. Basically the question is, what does metal gas look like microscopically? For example Mercury - with relatively low boiling/melting points and high vapour pressure - exists as a liquid at room temperature because its 6s electrons don't mix very well (something like that). But what does Hg vapour look like microscopically - are the gas particles just Hg atoms, or does the air mix with grains of Hg (clumped atoms) or dare I say Hg forms some bond with itself? I'm quite sure metallic bonding breaks down for gases, and that metals do not generally bond covalently. I know the Mercury cation is diatomic (Hg_2 2+), but is Mercury gas then just diatomic ions? Diatomic metal gases... Lastly, because of anomalies like hydrogen bonding in water and expansion under freezing (Beryllium i think), is it incorrect to say particles are more tightly packed in a solid than liquid?