I see that "pevný" can mean "solid" or "strong" or "stable". When talking about metals, any of those three could be said, but they all mean something different in English. For example:
1. Uranium is solid but not stable.
2. Sodium is solid but not strong.
3. Mercury is not solid at room temperature but is stable.
How does Czech make these distinctions?
Stable is stabilní. I think your strong in this case is the same as hard so tvrdý (vs. soft - měkký). At least that's what I imagine when you say sodium. If you mean strong as in stiffness, that is pevný.
Solid would be pevny as in strong and as in solid state of matter.
I am not a chemist but I believe that the 'stable' part would be 'stabilní'. There is also "Těkavý" but that is more of a "volatile".