alleryStart learning →What is the oxidation number of aluminium in AlH3?
AMinus three
BPlus three
CPlus one
DMinus one
Answer & Solution
Correct answer: B. Plus three
1. Aluminium is a metal, so the hydrogen bonded to it takes the value -1.
2. Three hydrogen atoms contribute 3 times -1, which is -3.
3. AlH3 is neutral, so the sum must be zero.
4. That gives 0 = x + (-3), so x = +3 for aluminium.
5. Using +1 for hydrogen here is the trap, and it would wrongly give -3 for aluminium.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 4 "Stoichiometry of Chemical Reactions", section 4.2 Classifying Chemical Reactions_
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