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Does Helium or Hydrogen have the greater atomic radius?

  1. Oct 28, 2013 #1
    I've been looking online at multiple resources and seem to be getting conflicting answers.

    http://www.sciencegeek.net/tables/AtomicRadius.pdf - Where He is slightly smaller than H

    http://intro.chem.okstate.edu/1314F00/Lecture/Chapter7/ATRADIID.DIR_PICT0003.gif - Where H is smaller than He

    http://environmentalchemistry.com/yogi/periodic/atomicradius.html [Broken] - Where H is bigger than He

    I could keep posting more sources but I think you get the gist of it. Also, the numbers from each source tend to vary and is this because of uncertainty or is another value (ex. covalent radius) being measured instead?

    Any help in discerning which atom, He or H, has the greater atomic radius and a brief explanation would be greatly appreciated!
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Oct 28, 2013 #2


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    Helium has smaller radius than hydrogen because in helium the electrons are attracted by a larger nuclear charge that is not fully "screened" by the electron-electron interaction. As far as I know, it's not possible to measure the covalent radius of helium, as it does not form sufficiently stable compounds. The radius has to be measured by other means or calculated quantum mechanically.

    I think the source claiming larger atomic radius for helium is erroneous.
  4. Oct 28, 2013 #3


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    A lot depends on the definition of atomic radius. Helium is a noble element (no known compounds) and never solidifies under a normal pressure, so even deciding what we will call the atomic radius is difficult. Most likely different sources use different definitions, so they get different results.
  5. Feb 22, 2017 #4
    Why is it that on some Atomic radius vs Atmoic number graphs, hydrogen has a smaller atmoic radius then helium? Please provide sources
  6. Feb 23, 2017 #5


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    Have you read posts above?
  7. Feb 26, 2017 #6


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    Borek's answer is valid not only for H and He but for any atoms, or indeed molecules. They are not 'hard spheres' and different types of experimental measurements will be sensitive to different funtions of the electron distribution in them.
    Last edited: Feb 28, 2017
  8. Feb 28, 2017 #7


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    By mole fraction, there is more He, (5.24 ppm), in air than H2, (0.55 ppm). If a vacuum tube gradually fills with hydrogen that enters through the glass envelope, (a molecular sieve), then a single He atom must be bigger than 2H as an H2 molecule.

    What gases do enter vacuum tubes over time ?
  9. Feb 28, 2017 #8


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    Interesting point, but I am not convinced it is a valid argument. There is a lot of additional effects (not related to simple geometry) responsible for the diffusion speed.
  10. Aug 6, 2017 #9
    In our school, relatively the noble gases are given the highest atomic radius by default; the reason being it isn't possible to measure the covalent radius so they take into account the Vanderwall radius which by definition is going to be higher than metallic or covalent radius, so yeah I guess it depends on the experimental approach
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