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In IIT-JAM 2018, There was one question: "which one of the following oxides are ionic?" I have selected $\ce{PbO}$ but according to the official answer key, $\ce{PbO}$ isn't ionic.

Although I know this distinction between ionic and covalent isn't defined sharply. But lead is a metal and oxygen is a non-metal. So according to me, it should be ionic. I tried to find on the internet I couldn't able to find out about $\ce{PbO}$, although $\ce{PbO2}$ is defined in between of ionic and covalent according to this paper. $\ce{PbO}$ should be more ionic than $\ce{PbO2}$ since oxidation state of the lead is lower, it will be less electronegative.

Even $\ce{PbCl2}$ is considered as Ionic according to many books, where chlorine is less electronegative and much bigger in size as compared to oxygen, So at least $\ce{PbO}$ should have more ionic character than $\ce{PbCl2}$ according to the Fazan's rule.

So should I challenge the official answer key? If yes, then on what basis? Which reference should I send to them?

enter image description here

Iono-Covalent Character of the Metal–Oxygen Bonds in Oxides: A Comparison of Experimental and Theoretical Data

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  • $\begingroup$ Be careful about punctuation, there shouldn't be any spaces before commas. As far as question is concerned, there shouldn't be such option as this isn't clear case, but I'm afraid that you're wrong and they're right. $\endgroup$
    – Mithoron
    Feb 27 '18 at 21:07
  • $\begingroup$ @Mithoron but according to DavePhD PbO is ionic. So how is it wrong ? $\endgroup$ Feb 28 '18 at 5:24
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    $\begingroup$ Haha, this is getting silly, both this question and such distinction are pretty much pointless. With one calculation you might get, say 47% of ionic character, with another mayyybe 53%, who knows? And what difference it would make? Would you actually know more about this compound? See for ex. chemistry.stackexchange.com/a/17072/9961 $\endgroup$
    – Mithoron
    Feb 28 '18 at 16:21
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If you want a reference to challenge the question, try page 248 of Advanced Study Guide Chemistry. There is a table on that page that says the PbO is "ionic".

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There are lots of different thoughts about qualitative assignment of whether $\ce{PbO}$ is ionic or covalent.

So, I think it will be better to approach it quantitatively and for that we can take help of Hannay-Smith's equation for calculation of ionic character in any binuclear compound.

% Ionic Character = [ 16 ($ \Delta\ $E.N.) + 3.5 ($ \Delta\ $E.N.)$^2$ ]

So, according to Pauling scale of E.N., we have E.N. of $\ce{Pb}$ = 1.87 (thanks to @Mithoron for correcting me) and E.N. of $\ce{O}$ = 3.44.

Now, putting the values we will get the % ionic character of $\ce{PbO}$ to be 33.75%, which, unfortunately for you and me (I also appeared in this year IIT-JAM and submitted $\ce{PbO}$ to be ionic),indicates that $\ce{PbO}$ is predominantly covalent in character.

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  • $\begingroup$ Pauling's e. value is much lower for Pb (+2), 1,87 according to en.wikipedia.org/wiki/Lead $\endgroup$
    – Mithoron
    Feb 28 '18 at 16:12
  • $\begingroup$ @Mithoron Yes, but in Hannay Smith equation, the e.n. of 'neutral atoms' are considered and not of them with their oxidation states. And I know you will now ask for ref. which unfortunately I can't get at the moment, but I think the example in this link ( google.co.in/… ) will help you understand that the e.n. of neutral atoms are under consideration! $\endgroup$
    – chail10
    Feb 28 '18 at 17:06
  • $\begingroup$ sigh Electronegativity depends on oxidation state for lead on +2 oxidation state it's 1,87, for Pb on +4 it's higher. Bare non-molecular ions have no electroneg. whatsoever. $\endgroup$
    – Mithoron
    Feb 28 '18 at 17:11
  • $\begingroup$ @Mithoron Obviously, that was too ignorant of me! I just got carried away with this ( google.co.in/… ) and tried to pile a logic upon that! I am correcting my answer. Thank you! But I doubt where that value 2.33 came from? $\endgroup$
    – chail10
    Feb 28 '18 at 17:30
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    $\begingroup$ I don't understand the purpose or the advantage of using an empirical equation, which is itself based on another empirical property (electronegativity), to try to obtain a precise quantification of percent ionic character. $\endgroup$
    – orthocresol
    Jun 24 at 8:43
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One of the greatest errors in all of science is the well-propagated notion that there are "ionic" and "covalent" bonds, as if a bond were always one or the other, and that we can tell them apart by electronegativity difference.

Neither notion is true.

  • Bonds always have a mixture of ionic and covalent contributions, even where they are between equivalent atoms like $\ce{H2}$. Nonpolar bonds as in molecular hydrogen have small, opposing ionic contributions that cancel each other out.

  • Bonds can show much greater ionic character than would be expected from the electronegativity differential. In many cases, a strong ionic bonding character is driven by molecular-orbital structure rather than electronegativity. For example, in magnesium diboride, the boron forms macromolecular structures with low-energy vacant orbitals and thus tends to become anionic (see here).

My answer to this question is to get a better textbook when one is available.

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