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First the question gives no units or that these are covalent radii or what, so I assume they are in nanometers and assume an atomic radius. I check my periodic table app and it says a 92pm atomic radius, 75pm covalent and 155pm van der waal for the N atom, which happens to be the first option I crossed out. Now what? BTW, this question is from a previous national aptitude test.

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  • $\begingroup$ If you remove the outermost electrons to give the ion do you think the atomic radius goes up or down? $\endgroup$ – Waylander Oct 24 '18 at 17:12
  • $\begingroup$ In problems with self-contained data you should look for consistency, not "truth." $\endgroup$ – MaxW Oct 24 '18 at 19:07
  • $\begingroup$ @Waylander of course the radius goes down, even I know that. But the question asks about adding electrons not removing them. $\endgroup$ – Sillysack Buttowski Oct 26 '18 at 7:57
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Effectively, the question is giving the radii in some sort of reduced units, where it's expressing the size of each species relative to the size of something else.

Thus one can interpret the choices in this question as providing relative sizes. So, for instance, choice "A" says the ion is 2.60/.77 = 3.38 times the size of the neutral atom, and so on.

Given your knowledge of the actual sizes of the ion and the neutral atom, you should now be able to choose the best answer.

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  • $\begingroup$ But I don't know the size of the $\ce{N-3}$ ion. $\endgroup$ – Sillysack Buttowski Oct 26 '18 at 7:53

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