Electronegativity of H is greater than P, but according to my book in PHX3, P has oxidation number of -3. How is it possible? Is any of the data wrong?
Answer
Here is a way to get the answer. If you place a reagent with oxidation number of H = -1 (hydride) it would produce dihydrogen gas. For example: KH+HX2O⟶KOH+HX2.
If you place a molecule with oxidation number of H=+1 dihydrogen gas is NOT formed. NHX3+HX2O⟶NHX4X++OHX−. Phosphine doesn't form dihydrogen gas on contact with water so H is in oxidation state +1 and P is in oxidation state -3. Reaction with strong acids leads to PHX4X+ which has similar structure to the ammonium ion. For these reasons even for As in AsHX3 the oxidation state of As=−3.
The problem with electronegativity is that we are trying to assign a single number, while we actually need a function. We take average of electronegativity of H in HF,NaH,HX2O,CHX4,NHX4X+ and come up with a single number. This works well if you need to guesstimate the behavior, but it is not accurate. It doesn't even account for softness/hardness of a partner.
Analysis "will PHX3+HX2O form HX2?" (no) vs "will PHX3+PX2OX3 form P+HX2O?" (yes) is more accurate way to assign formal charges.
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