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In the periodic table of the elements, only a few of the elements are gases at standard temperature and pressure. Those elements include the noble gases some of the halogens, and a few of the elements next to them, i.e. oxygen and nitrogen.

So why are these elements gases?

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You could just as well turn this around and ask why STP lies in a range where the elements and common compounds comes in all three "usual" states of matter. And then the answer takes on the aspect of the weak anthropic principle. – dmckee♦ Feb 4 '11 at 0:30
Can't say I understand the downvote. The question has an objective answer at the border of physics and chemistry. – dmckee♦ Feb 4 '11 at 1:27
The downvote is for the hint to that closed "Why are the alkali...." – Georg Feb 4 '11 at 1:39
Downvote revoked. – Georg Feb 4 '11 at 23:10

2 Answers

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The [EDIT] lighter [/EDIT] halogens are quite eager to form covalent bounds and become diatomic molecules. These are strongly bound. Because charge is distributed symmetrically among these bounds, the resulting molecules are not strongly polarized, so the interaction with other molecules is relatively weak. Therefore, you neither have solids nor liquids with them.

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Ever heard of Iodine, a solid? Or from Bromine, a liquid? -1 – Georg Feb 4 '11 at 1:14
But those aren't the light ones, are they? – Lagerbaer Feb 4 '11 at 3:12
Light halogens are the most electronegative elements in the periodic table en.wikipedia.org/wiki/Electronegativity so their bonds are among the strongest ones the elements may exhibit. That's why they instantly form the diatomic molecules, $F_2$ and $Cl_2$. After all, the same is true for nitrogen and oxygen next to fluor in the periodic table, too. The resulting diatomic molecules are symmetric and still "electronegative", i.e. "thirsty to steal e-", but in the fluor or chlor gas, there is no one who could donate electrons so the molecules don't grow and don't interact with each other. – Luboš Motl Feb 4 '11 at 7:03
Your Comments reads: "The halogens are ..." doesn't it? – Georg Feb 4 '11 at 10:31
Another two errors: the bonds in F2 and Cl2 are rather weak compared to bonds in O2, N2 and H2 ! ( that is why they are so reactive!)Next: ".. not strongly polarized..." Such symmetric molecules are not at all polarized! Boiling point is "caused" by dispersion forces only. @Lubos: Electronegativity gives some hint about polarization of bonds between different elements, there is almost no connection to the binding strength. – Georg Feb 4 '11 at 17:18
up vote -1 down vote

It is basically for the reason Lagerbaer states. They have one open p-orbital available and bind in diatomic molecules. The perturbation on the other closed p-orbitals is small so there is little dipolar interaction. You have to make then very cold to get a liquid.

I don’t know what liquid Cl would be like, but something tells me it is a nasty fluid.

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Liqid Cl2 is in all the steel cylinders where Cl2 is written on. (Including those at Ypern). And a typical railway tank wagon carries up to 20 tons of liquid Cl2. – Georg Feb 4 '11 at 1:17
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This answer is unfortunate in a bunch of ways. But it should be noted that Germany accounted for only about half the poison gas production in the first world war. – Carl Brannen Feb 4 '11 at 2:54

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