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$\ce{F2}$ 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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    $\begingroup$ Nevermind 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. $\endgroup$ – J... Aug 14 '17 at 11:44
  • $\begingroup$ Or when it comes to naturally appearing elements, just think of radon. $\endgroup$ – Vladimir F Aug 14 '17 at 13:03
  • $\begingroup$ @J... If I'm doing my math right, Tungsten Hexafloride is more dense than aluminum! Thanks for showing me amusing compounds! $\endgroup$ – Cort Ammon Aug 14 '17 at 16:52
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    $\begingroup$ @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. $\endgroup$ – J... Aug 14 '17 at 16:57
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    $\begingroup$ So the reason Sulfur is solid (and not diatomic) and Oxygan is gas diatomic is difference in electron configuration? $\endgroup$ – JeopardyTempest Aug 15 '17 at 17:13
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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.

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    $\begingroup$ Excellent physical analogy with the bricks & mortar. I will totally be swiping it. $\endgroup$ – hBy2Py Aug 14 '17 at 12:10
  • $\begingroup$ 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)? $\endgroup$ – DrZ214 Aug 14 '17 at 21:29
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    $\begingroup$ Under ordinary circumstances, yes. There are, as usual, some extra-ordinary circumstances that can produce different outcomes. $\endgroup$ – Puppy Aug 14 '17 at 22:15
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    $\begingroup$ @DrZ214 Have a read - Why fluorine is a gas. Further reading - Van der Waals Force. $\endgroup$ – J... Aug 14 '17 at 23:36

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