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In IIT-JAM 2018, There was one question, which one of the following oxides are ionic? There was one option PbO also. According to the official answer key, PbO isn't ionic.

Although I know this distinction between ionic and covalent isn't defined sharply. But Pb 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 PbO, although PbO2 is defined in between of ionic and covalent according to this paper. PbO should be more ionic than PbO2 since oxidation state of the Pb is lower, it will be less electronegative.

Even $\ce{PbCl2}$ is considered as Ionic according to many books, where Cl is less electronegative and much bigger in size as compared to O, So At least 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 ? $\endgroup$ – Aditya Shrivastav 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
  • $\begingroup$ There are many different electronegativity scales. If I try to use Hannay-Smith's equation using Pauling electronegativity scale then Al2O3 is also covalent. Al(III) has an electronegativity of 1.61 on Pauling scale. PIC of Al2O3 is 41 %. But according to IITB Al2O3 is ionic but PbO isn't. How unfair !! Moreover, the electronegativity value of Al(III) and Pb(II) is almost same on Allred Rochow electronegativity scale, i.e. 1.47 and 1.50 respectively. $\endgroup$ – Aditya Shrivastav Feb 28 '18 at 18:32

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