- Hyrdrium. Just how? I mean it's said to be lighter than hydrogen, which unless it's using a completely different hadron as a nucleus is just silly.
- Magic. But more seriously, if the individual atoms repelled each other strongly enough (say, it somehow became and remained ionized) the gas laws would not adequately describe its behavior and it could have a lower density (not mass, but most people wouldn't recognize the distinction) than would be predicted at a given temperature and pressure.
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