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Why is the correct option O^2-, aren't all of them equal since they all have the same number of energy shells and same number of electrons in them

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  • $\begingroup$ Let's start with a simpler question. $\rm H$ (neutral) and $\ce{He+}$: what's bigger? They have the same number of electrons (one). $\endgroup$ Feb 18, 2023 at 12:57
  • $\begingroup$ Well, 2- ions rapidly become 1- ions, so I consider it a trick question. $\endgroup$
    – Jon Custer
    Feb 18, 2023 at 13:28
  • $\begingroup$ @JonCuster Unless there is meant a ionic radius in ionic compound context. $\endgroup$
    – Poutnik
    Feb 18, 2023 at 13:39
  • $\begingroup$ Sure, context matters there. $\endgroup$
    – Jon Custer
    Feb 18, 2023 at 13:42

1 Answer 1

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F-: There is only one electron added, there is inter-electronic repulsion due to which the radius increases

Mg2+: Two electrons are removed, but the number of protons remain the same, so nucleus pulls the electrons towards itself, and the radius decreases slightly

Na+: An electron is removed, so there is more effective nuclear charge, and pulls electrons towards nucleus, so again radius decreases.

O2-: Two electrons are added there is more inter-electronic repulsion, so the radius increases (and it is more than F-, because here two electrons are added but in F- only one electron was added)

In these questions where we compare the radius of isoelectronic species, i.e. species with same number of electrons, we compare the ratio of Nuclear charge (Z) to the number of electrons(e) ratio.

If Z/e ratio increases the radius decreases,and if ratio decreases radius increases.

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