help i have chem test tomorrow and i forgot the electronegativity difference threshold for ionic bonds, some places say 1.7 others say 1.9 and some say 2.0 so i figure i ask vlr
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high school????? did not expect that. heres my advice as a chem major, forget everything you know abt chem
and then never take it again
i wanted to take ap chem at some point later on :/
i had just forgotten this one thing but i got everything else for the test, got a 99 last marking period
Googled it and found this on Reddit:
[This is literally the most tragic and biggest misconception in chemistry where students think electronegativity difference alone determines ionicity vs covalency. 1.7 / 1.8 / whatever value should not be the black-and-white line that you draw between the two extremes, as there are many exceptions, and wrong conclusions could be drawn if you resort to your approach.
Examples of common exam-exceptions
- BF3: is a very common IB molecule, and is a gas and exists as simple molecules even though the B-F EN difference (2.0) says otherwise
- Silver compounds: silver has a high electronegativity as far as metals go, and silver-anything other than fluoride will not cross the 1.7 boundary. Yet most silver compounds are treated as ionic compounds
- transition metals: theser are not IB-memorization examples, but Cr2O3 is mostly ionic (mp over 2000 oC) and CrO3 is mostly covalent (acidic oxide)
The proper way to use electronegativity on exams is "the highER the EN difference, the MORE polar the bond", but the EN difference alone does not directly indicate ionicity vs covalency, which is determined by experimental / physical data/ chemical properties](https://www.reddit.com/r/IBO/s/ShmKJLqyTl)
this is actually really good advice, just memorizing the electronegativity threshold values as a way to determine the bonds will come back to bite you when you get to all the exceptions
especially considering op wants to take ap chem, it's better to learn chemistry conceptually rather than through strict rulesets
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