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Atomic/Ionic Radii: Helium vs. Lithium+ (ion)

  1. Apr 11, 2012 #1
    I'm teaching periodic trends in a Gen. Chem. course, and it's commonly mentioned that the radius of an Li+ ion (90 pm) is smaller than that of the neutral Li atom (~160 pm), with the dominant reason being that the valence 2s1 electron was removed. The same trend shows up for the other alkali metals, naturally.

    However, the ionic radius of Li+ is 90 pm, while the atomic radius of He is 31 pm (calculated). The strange thing (to me, anyway) is that they both have the same electron configuration, namely two electrons in the 1s orbital. Li+ has the larger nuclear charge, so I'd have expected Coulomb's law to pull those two electrons closer to the nucleus, but apparently the exact opposite result happens.

    Anyone know what I'm missing? Similar trends seem to be observed for the other pairs of alkali ions/noble gases...

  2. jcsd
  3. Apr 11, 2012 #2
    This is an itneresting observation. I will throw a hint but I do not claim that I have a definite answer right now. My understanding is that the ionic radius for all elements is "estimated" in and very "crude" way when this element is part of an ionic crystal. My guess that in an ionic crystal the periodic electric field will affect the distribution of the electron "cloud" around the nuclei in an unintuitive way. Other unintuituve examples can be found by examining this table for ionic radii:
    For instance Nb3+ can have ionic radius less than Li+ although the atomic number of Nb is 41.

    EDIT: I also think it is unfair to compare ionic radii with atomic radii. Comparison and trends are meaningful only when the radii compared are of the same type. For example, it looks absurd to me to compare the van der Waals radius of Si with the ionic radius of Mn.
    Last edited: Apr 11, 2012
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