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 March 26th, 2017, 03:52 AM #1 Member   Joined: May 2016 From: Ireland Posts: 96 Thanks: 1 ideal gas Out of the gases hydrogen, nitrogen, chlorine, and helium, which shows behaviour closest to an ideal gas? I've been told the answer is helium because there are no attractive forces between helium atoms. Would there not be Van der Waals forces though? Also, would hydrogen just have as little attractive forces as well? Last edited by skipjack; March 26th, 2017 at 08:39 AM.
 March 26th, 2017, 04:51 AM #2 Senior Member   Joined: Jun 2015 From: England Posts: 891 Thanks: 269 Van der Waals forces arise because the atoms or molecules are 'unbalanced' or unsymmetrical in their electrical distribution in space. They are still electrically neutral, it does not mean that they are electrically charged. It means. We consider the positive nucleus as fixed and the negative electron or electrons as whizzing around the nucleus. As they whiz around, if there are an even number of electrons, the individual attractions between the nucleus and the electrons cancels out on average. But if there are an odd number of electrons this is not possible. Hydrogen has one electron, helium has two. But this is only one part of the reason for the deviation from the ideal gas laws. The other part is due to the fact that the atoms/molecules occupy physical space. The bigger this space, the greater this deviation component is. Van der Waals introduced a correction factor in his equation of state for this as well. Last edited by skipjack; March 26th, 2017 at 09:05 AM.
 March 26th, 2017, 08:06 AM #3 Member   Joined: May 2016 From: Ireland Posts: 96 Thanks: 1 So which is it - hydrogen or helium? More temporary dipoles form in helium, so it's less close to an ideal gas. Because hydrogen has only 1 electron so it's smaller, so molecules are more widely spaced. Therefore it's closer to being an ideal gas. The right answer is helium though. Now I am even more confused than originally. Last edited by skipjack; March 26th, 2017 at 08:33 AM.
 March 26th, 2017, 08:46 AM #4 Senior Member   Joined: Jun 2015 From: England Posts: 891 Thanks: 269 Hydrogen is a molecule, which is larger than a helium atom. Helium does not readily form molecules, even with itself.
March 27th, 2017, 03:38 AM   #5
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 Originally Posted by markosheehan So which is it - hydrogen or helium? More temporary dipoles form in helium, so it's less close to an ideal gas. Because hydrogen has only 1 electron so it's smaller, so molecules are more widely spaced. Therefore it's closer to being an ideal gas. The right answer is helium though. Now I am even more confused than originally.
I'm not sure I agree that hydrogen behaves more like an ideal gas, because it can form diatomic hydrogen (H$\displaystyle _2$), therefore giving it a vibrational degree of freedom.

Helium is a noble gas. The noble gases are, broadly speaking, the most unreactive of all the elements and very rarely participate in chemical reactions. They always have exactly enough electrons to fill their outer electron shells, so they are already in their most stable state with regards to the possibility of combination with other atoms of the same kind.

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which of the noble gases is nearest to being an ideal gas

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