$\ce{F2}$ is a gas, but pure lithium is a solid metal. Why is this? Lithium is much lighter, just 3 protons and 3.9 neutrons (average). Fluorine is much heaver, 9 protons and 10 neutrons.
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23Nevermind F2 - how about Tungsten Hexafluoride? It's 13 times denser than air and has a molecular weight of almost 300g/mol. Being a gas has very little to do with how heavy the atoms or molecules are - it's entirely about how sticky they are. – J... Aug 14 '17 at 11:44
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Or when it comes to naturally appearing elements, just think of radon. – Vladimir F Героям слава Aug 14 '17 at 13:03
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@J... If I'm doing my math right, Tungsten Hexafloride is more dense than aluminum! Thanks for showing me amusing compounds! – Cort Ammon Aug 14 '17 at 16:52
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2@CortAmmon Definitely not more dense than aluminum. WF6 is 13g/L (or 13mg/cm3). Aluminum is 2.7g/cm3 (or 2700mg/cm3) - about 200 times denser than WF6. Still, it's a pretty heavy gas. You could fill a fishtank with it and float aluminum foil boats on top. If you look around youtube you'll find interesting experiments with SF6, which is also a very dense gas. – J... Aug 14 '17 at 16:57
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@J... Ahh thank you. I thought it was too good to be true (I had copied the wrong density from Wikipedia's page) – Cort Ammon Aug 14 '17 at 17:05
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@J... It would make a worthy question on this site. For now, I will stick to F2 vs Lithium and worry about more complex chemicals later. If you have an answer, please post it because I still don't understand. – DrZ214 Aug 14 '17 at 21:27
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@J... I'm certainly no chemist, but wouldn't it therefore indirectly indeed have to do with mass, since lighter molecules have more velocity for the same temperature, and so tend to be "less sticky"? And that'd explain the propensity for the lightest atoms/molecules to trend towards gaseous? – JeopardyTempest Aug 15 '17 at 13:03
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@JeopardyTempest No, not really. It's more about the electron configuration. Electrostatic forces dominate by far at this level. You can't think about this like popcorn or billiard balls. – J... Aug 15 '17 at 13:07
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1So the reason Sulfur is solid (and not diatomic) and Oxygan is gas diatomic is difference in electron configuration? – JeopardyTempest Aug 15 '17 at 17:13
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@JeopardyTempest Yes, largely. – J... Aug 15 '17 at 18:17
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A lithium atom has one valence electron, easily lost (shared), so it is connected to other atoms by a metallic bond. This is a bit like the shell game where a pea (electron) is hidden under a walnut shell... the uncertainty of where it is at any instant implies, in a quantum sense, that all the atoms share it, and are held together. This bond is so strong that the next element, beryllium, has a boiling point of $\pu{3243 K}$.
Fluorine, on the other hand, has an almost complete shell and each atom forms a tight covalent bond with just one other fluorine atom. These molecules of $\ce{F2}$ keep their electrons to themselves and do not associate with each other.
The difference in bonding is like the difference between bricks mortared together in pairs, or mortared together in a three-dimensional structure, and is responsible for the difference in mp and bp.
andselisk
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DrMoishe Pippik
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5Excellent physical analogy with the bricks & mortar. I will totally be swiping it. – hBy2Py Aug 14 '17 at 12:10
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This bond is so strong that...So are you saying, in general, metallic bonds are much stronger than covalent bonds?These molecules of F2 keep their electrons to themselves and do not associate with each other.And are you saying that one F can only bond to one other F, while Li can bond to many other Li (which would make a large heavy crystal)? – DrZ214 Aug 14 '17 at 21:29 -
2Under ordinary circumstances, yes. There are, as usual, some extra-ordinary circumstances that can produce different outcomes. – Puppy Aug 14 '17 at 22:15
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2@DrZ214 Have a read - Why fluorine is a gas. Further reading - Van der Waals Force. – J... Aug 14 '17 at 23:36