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Hydrogen bonding requires hydrogen bonded to a highly electronegative atom that also:
Acarries at least one lone pair of electrons
Bholds a full positive formal charge
Cbelongs to the third period or lower
Dforms at least two double bonds
Answer & Solution
Correct answer: A. carries at least one lone pair of electrons
1. Two conditions must both hold for a hydrogen bond to form.
2. First, hydrogen must be covalently bonded to a small, highly electronegative atom — nitrogen, oxygen or fluorine.
3. Second, that electronegative atom must carry at least one lone pair, which is what the neighbouring hydrogen is attracted to.
4. Water qualifies because oxygen has two lone pairs; ammonia qualifies because nitrogen has one.
5. Missing the lone-pair condition is why candidates could not explain which molecules hydrogen bond and which do not.
_Source: NECTA ACSEE 2023 Chemistry 132/1, Question 3: Chemical Bonding_
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