[reed]

Hydrogen, Nitrogen, Oxygen, and other elements form diatomic molecules with themselves. Phosphorus and Sulfur form weird molecules that are polyatomic - S₈ and P₄. Why don't metals form these diatomic molecules?

Another question:
My book claims that the compound UF₆ is molecular. But the electronegativity of Fluorine is 4 and U is 1.4. A difference in their electronegative is 2.6 which would make it an ionic compound and not molecular. Why does my book claim it's molecular?

[Arkcon]

Since metals tend to share their outermost electrons across many atoms, it would be hard to say where one molecule ended and another began, so the convention is to just not use numbers in it's molecule, when writing a balanced equation, since the number of atoms in the molecule varies with the allotrope.  Sometimes, when writing a sulfur, or phosphorus reaction, people will just leave off the coefficient for the molecule.  Likewise, iodide forms a diatomic molecule, in the vapor phase, but for iodine crystals, it is hard to know where one pair would end, and another begin, so sometimes, we leave it off.

[reed]

Thanks, but why does Uranium form a molecule if it's a metal?

[Arkcon]

Thanks, but why does Uranium form a molecule if it's a metal?

See, now that I don't know, but lookie here: a similar topic has come up on these boards recently
http://www.chemicalforums.com/index.php?topic=30524.msg116030#msg116030

[Vidya]

metals have tendecy to lose electrons and they form metallic boding in which  electrons have high mobility in three dimension over all atoms in the crystal.So whatever no of atoms  are present in the crystal ,they all are bound to each other due to these free mobile electrons.

whereas in non-metals covalent bonds are present in molecules with fixed no of atoms.
I think this will help you to understand the concept.
UF6 is a octahedral molecule in which U is +6 oxidation state if it forms complete ionic bond with 6F.Any atom with +6 O.S itself becomes highly electronegative .So I think that can be reason for UF6 to have molecular structure .