Its existence was predicted in 1935 and it’s still pretty theoretical. But it’s pretty much the only thing that could explain Jupiter’s magnetic field. Just last year experimentalists claimed to have produced metallic hydrogen in a diamond anvil, but those results are under some scrutiny and AFAIK haven’t been reproduced yet.
I've seen a quick-and-dirty summary of how to think about bonds that goes like this:
- An ionic bond forms between two atoms, one of which has too many electrons and one of which has too few. They balance.
- A covalent bond forms between two atoms, both of which have too few electrons. Some electrons are shared between both atoms simultaneously, bringing each up to quota through the simple magic of double-counting.
- A metallic bond forms "between" some number of atoms which all have too many electrons. The surplus electrons roam back and forth freely around the metal because their home atoms would prefer them to go away, allowing the metal to carry electric current.
This model is supposed to explain why you can basically combine arbitrary metals in arbitrary ratios and still get a metal.
It also seems like it predicts the existence of metallic hydrogen, in that a normal H ion is positive. Does that actually make sense?
It makes sense from the periodic table, but I really didn't know this existed. Very cool!