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In an exam paper it says that the lattice energy of NaCl is more exothermic than MgCl as the Na+ cation has a smaller ionic radius than Mg+. Here are the resources given:enter image description hereenter image description here

What is the reasoning for for Na+ having a smaller ionic radius even though Mg+ has a higher proton number?

Thanks.

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    $\begingroup$ Mg+ is not a thing at all. $\endgroup$ – Ivan Neretin Oct 16 '18 at 4:47
  • $\begingroup$ It was part of a CIE A - level chemistry exam $\endgroup$ – Devansh Shah Oct 16 '18 at 5:14
  • $\begingroup$ I'd agree with Ivan. You must be making a mistake. It should be $\ce{Mg^{2+}}$ and $\ce{MgCl2}$. $\endgroup$ – MaxW Oct 16 '18 at 5:51
  • $\begingroup$ In the exam I think it was asked as hypothetical situation, so hypothetically how would you reason the ionic radius of Mg+? $\endgroup$ – Devansh Shah Oct 16 '18 at 5:55
  • $\begingroup$ We wouldn't. It makes no sense. $\endgroup$ – Ivan Neretin Oct 16 '18 at 6:01
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We actually do mean $\ce{Mg^+}$ here. The idea is to show what factors are involved when an alkaline earth metal forms stable compounds with $\ce{M^{2+}}$ ions whereas an alkali metal favors $\ce{M^+}$. In this case, as pointed out in some of the comments, $\ce{Mg^+}$ is bulked up by the electron that remains in the relatively diffuse, loosely held $3s$ subshell. You need to remove that electron, making $\ce{Mg^{2+}}$, to get a compact ion that gives good lattice energies in ionic crystals.

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    $\begingroup$ Just out of interest Magnesium(I) compounds are known, but they are of the form [Mg-Mg]2+ rather than a bare Mg+ - so more like Mercury than Sodium. See pubs.rsc.org/en/content/articlelanding/2011/dt/… , and in a way this furthers the similarity between Mg and Zn. $\endgroup$ – Ian Bush Oct 16 '18 at 10:24
  • $\begingroup$ Not just magnesium. Wikipedia also documents similar dimers for other alkaline earth metals. Such dimers, of course, are easily oxidized to +2 monatomic ions. $\endgroup$ – Oscar Lanzi Oct 16 '18 at 11:14

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