How would you determine the size of a polyatomic ion?

For example when determining which ion is bigger, $\ce{NH2-}$ or $\ce{I-}$, what would you use to determine the size difference?

I know that $\ce{I-}$ would be much smaller than $\ce{I2-}$ (since the $\ce{e-}$ are not held as tightly in the $\ce{I2-}$ causing the repulsion to increase)

But what basis do we use to determine size of a nonelemental atom like the polyatomic ion or a more complex molecule for that matter?

  • 1
    $\begingroup$ Well, you'd need atomic/anionic radii, bond lengths and angles. $\endgroup$
    – Mithoron
    May 2 '15 at 18:42
  • 1
    $\begingroup$ better late than never i guess haha :P $\endgroup$
    – Nerdingout
    May 6 '15 at 19:30
  • $\begingroup$ Well, it's problematic question - how would you compare them - by volume, length, effective volume? $\endgroup$
    – Mithoron
    May 6 '15 at 21:04

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