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Generally, the percent of ionic character in a two-element compound correlates quite well with the difference in the electronegativities of the two elements making up the compound, as can be seen in this graph:

Ionic v. Covalent in the Electronegativity Court

(Image originally by the University of Florida, via user4076 here at ChemSE.)

Even here, though, an anomaly is visible.

  • HF has an electronegativity difference of 1.893 (4.193 - 2.300).
  • LiBr has an electronegativity difference of 1.773 (2.685 - 0.912).
  • KI has an electronegativity difference of 1.625 (2.359 - 0.734).
  • LiI has an electronegativity difference of 1.447 (2.359 - 0.912).

Based on this, HF should be mostly ionic - certainly more so than LiBr, KI, or especially LiI. Instead, judging from the graph, HF is slightly more covalent than ionic, with ~45% ionic character, whereas LiBr is ~60% ionic, KI is ~75% ionic, and even LiI is ~52-53% ionic despite having a considerably-lower electronegativity difference than the ~45%-ionic HF.

Going along the sequence KI - LiBr - HF, the ionic character of the compound actually declines steeply as the electronegativity difference gets somewhat greater.

What is going on here?

Vikki
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    Hints: Look up hydrogen fluoride and behold its extravagant chemical behavior. Also note hydrogen bonding is involved. Very involved! – Ed V Mar 15 '21 at 01:50
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    This calculation of values is funny - precision to one thousandth, really? You're treating stupid electronegativity way too seriously, and you're only proving that you shouldn't. – Mithoron Mar 15 '21 at 02:08
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    https://chemistry.stackexchange.com/questions/17064/is-kf-the-most-ionic-compound – Mithoron Mar 15 '21 at 02:11
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    Do you consider H+ would be just a proton/deuteron with the extremely strong electrostatic field ? – Poutnik Mar 15 '21 at 07:23
  • @Poutnik Well, in the optics of OP an ionic bond to something would fix that. – Alchimista Mar 15 '21 at 09:30
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    Does this answer your question? Is KF the most ionic compound? – Alchimista Mar 15 '21 at 09:35
  • @Alchimista: No, it doesn't. – Vikki Mar 16 '21 at 00:13
  • @Mithoron: All the electronegativities in question are available with that degree of precision, so why not? – Vikki Mar 16 '21 at 00:14
  • @EdV: That still doesn't explain KI v. LiBr. – Vikki Mar 16 '21 at 00:14
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    In my judgement, no need: the table is pretty much a waste. Chemistry has lots of trends, patterns and exceptions. Electronegativity is only one factor that, unfortunately, gets over-emphasized in early chemistry courses. And there are several ways to define electronegativity, leading to arguments such as “Is nitrogen more electronegative than chlorine or vice versa?” – Ed V Mar 16 '21 at 00:29
  • @EdV: Except that difference-in-electronegativity is directly linked to bond polarity and ionic character. – Vikki Mar 16 '21 at 00:45
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    So why is the plot not a straight line if the difference in EN is the sole factor? I will stay with what I said. – Ed V Mar 16 '21 at 00:57
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    "difference-in-electronegativity is directly linked to bond polarity and ionic character" in which electronegativity? Allen electronegativity you used has no direct connection to bonds at all. – Mithoron Mar 16 '21 at 01:21
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    @Vikki-formerlySean that is a bit surprising. In my opinion it should. If you stick to the fact that bigger the delta El Neg more polar the bond is, you won't obviously get out of this. – Alchimista Mar 16 '21 at 09:37
  • https://en.wikipedia.org/wiki/Fajans%27_rules – Mithoron Mar 18 '21 at 17:34

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