Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: ChinaCat on 05/01/2004 16:00:46

Title: Why is helium smaller than hydrogen ?
Post by: ChinaCat on 05/01/2004 16:00:46

[:D] Hi guys- I just stumbled onto this site and am hoping that someone can help answer this question:

Why is a helium atom smaller than a hydrogen atom?

I know that mass-wise, the H atom is smaller. Is the question referring to the amount of space that the He atom takes up???

Please help if you know-- this question was in a review guide for an exam I am taking this weekend and it's driving me nuts. I can't find a good explanation anywhere!

Title: Re: Why is helium smaller than hydrogen ?
Post by: Ylide on 06/01/2004 02:32:33

Along a period (left to right) the the atomic number increases while the valence electrons remain in the same shell. Thus due to the increasing nuclear charge (pulling electrons closer to the nucleus) the radii of the atoms decrease left to right.

And as you can see from the table, Hydrogen and Helium are in the same period.

Hope that helped.



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Title: Re: Why is helium smaller than hydrogen ?
Post by: Kryptid on 07/01/2004 01:41:25

Yes, cannabinoid summed that up correctly.

Data about the generallized radius of the helium atom is difficult to come by since helium does not bond with other atoms. If any measurement is given, it is usually either theoretical or is a measure of the "van der walls" radius, which is much different than the covalent radius used for many other atom's measures.