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Why does the density of the halogens increase as you go down Group 7?
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Corrie writes ......,
 
I presume you are comparing the halogens in their gaseous forms, otherwise we are dealing with a comparison between gases (fluorine and chlorine), a liquid (bromine) and a solid (iodine,) at room temperature, which obviously increase in density on account of the change of state, before considering other factors?
 
If we consider the halogens as all in the gaseous form, we can apply the rule that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules applies to the halogens in their gaseous form, even though they are not ideal gases. Then consider the molecular masses of the molecules X2 (where X = F -> I). You will see that there is a large increase in molecular mass as you go down the Group, accounting for the large increase in density of the gaseous halogens.

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updated: 29 April 2010

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