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). $\endgroup$– Ivan NeretinFeb 18 at 12:57
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$\begingroup$ Well, 2- ions rapidly become 1- ions, so I consider it a trick question. $\endgroup$– Jon CusterFeb 18 at 13:28
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$\begingroup$ @JonCuster Unless there is meant a ionic radius in ionic compound context. $\endgroup$– PoutnikFeb 18 at 13:39
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$\begingroup$ Sure, context matters there. $\endgroup$– Jon CusterFeb 18 at 13:42
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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.
