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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?

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    $\begingroup$ 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. $\endgroup$
    – Mithoron
    Mar 15, 2021 at 2:08
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    $\begingroup$ chemistry.stackexchange.com/questions/17064/… $\endgroup$
    – Mithoron
    Mar 15, 2021 at 2:11
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    $\begingroup$ Does this answer your question? Is KF the most ionic compound? $\endgroup$
    – Alchimista
    Mar 15, 2021 at 9:35
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    $\begingroup$ 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?” $\endgroup$
    – Ed V
    Mar 16, 2021 at 0:29
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    $\begingroup$ 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. $\endgroup$
    – Ed V
    Mar 16, 2021 at 0:57

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