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Electronegativity is the tendency of atoms in covalent bonds to attract electrons closer to themselves (I'll admit I realised ions do not form covalent bonds only after I finished writing).

Electronegativity depends on a variety of factors including the number of protons, the number of electrons in the outer shell and the ionic radii. In general it depends on the effective nuclear charge (or the pull)

Generally Atoms towards the left end of the Periodic Table are more electropositive (groups 1,2,3...) while those at the right end of the Periodic Table are more electronegative (group 17,16...). So $\ce{Cl}$ should be more electronegative than $\ce{Na}$.

Now coming to ions, it should be noted that in Chlorine the outermost shell is completely filled so its nearly impossible for Chlorine to continue attracting due to electron repulsion, so its EN must have reduced.

On the other hand, $\ce{Na+}$ is positively charged and so it should be attracting more electrons but isn't its octet already filled so should it be still attracting electron?

Is it incorrect to compare qualitatively the electronegativities of ions? I wasn't able to find much on the google about it.

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    $\begingroup$ Electronegativity is defined for elements, not for ions. $\endgroup$
    – Poutnik
    Apr 9, 2021 at 11:00
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    $\begingroup$ I would look at electron affinity and ionization energy instead, and read more on these subjects. $\endgroup$
    – Buck Thorn ♦
    Apr 9, 2021 at 11:01
  • $\begingroup$ @BuckThorn and thanks, but could you check the comments I made on goarkz answer? $\endgroup$
    – Adil Mohammed
    Apr 9, 2021 at 15:53

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Electronegativity is defined for an atom in a molecule. It directly means that we talk about electronegativity of chlorine we are talking about its behaviour with respect to its molecules in general.

As I understand what you are trying to compare looks more like comparing Electron Affinity (or Electron Gain Enthalpy), which is different from Electronegativity.

For example:

  1. E.N. of $C$ in $\ce{CH4}$ $<$ E.N. of $C$ in $\ce{CO2}$
  2. E.N. of $Cl$ in $\ce{Cl2O7}$ $>$ E.N. of $C$ in $\ce{Cl2O5}$

Greater the oxidation state, greater is the electronegativity.

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  • $\begingroup$ Thanks, but can you help me find Electron gain enthalpy. I found its value was -2.55 eV for Na+ and 3.7 eV for Cl-per atom at separate sources. Could you verify if possible? Also if it is true then is Na+ is much better at attracting electrons than Cl- as it is releasing energy when added and therefore more stable $\endgroup$
    – Adil Mohammed
    Apr 9, 2021 at 15:52
  • $\begingroup$ nope those are not for ions, only for the parent atom $\endgroup$
    – Adil Mohammed
    Apr 9, 2021 at 15:54
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    $\begingroup$ @AdilMohammed Firstly Electron gain enthalpy for Na+ is opp. of its Ionisation enthalpy, so for Na+ it is -496kJ/mol which is close to 5.1eV per atom. Secondly, if we blindly see them as +vely charged species and -vely charges species then yes they yes Na+ will "attract electrons" it means if there are sodium ions in water, lone pair of oxygen will get attracted to Na+ which is hydration of sodium ion. I hope you get the second point meaning, if not feel free to comment again $\endgroup$
    – gobbledy-gook
    Apr 9, 2021 at 16:01

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