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Grade: 12
Why does PH3 has an exceptional bond angle of 93.5?
10 months ago

Answers : (2)

22784 Points
Drago’s rule is basically a rule of hybridisation.
Conditions for dragos rule:
i.) no. of sigma bonds+ l.p.=4
ii.) the central atom must belong to 3rd, 4th, 5th or 6th group(not to 2nd)
iii.) the electronegativity of central atom must be \leq 2.5
iv.) Atleast 1 lone pair must be there on central atom.
if these conditions are fulfilled then the compound follows dragos rule, which says that there will be bo need to consider hybridisation.
drago’s suggested an empirical formula, which said that, in these cases, the bond angle is exceptionally small approx. 90~94 degree.And this is the reason why PHhas an exception bond angle of 93.5 degree
10 months ago
231 Points
From the Lewis Structure of PH3, we get its hybridisation to be sp3. Basically it has three bond pairs and one lone pair on P. However, since it matches the conditions of Drago’s Rule, it is a Drago Molecule and Drago Molecules do not have any hybridisation and have less bond angles. PH3 does not have any hybridisation because it’s bond formation is due to the overlapping of pure p-orbitals.
Now, if you study the reason of having less bond angle from the core:
PH3 has a Pyramidal shape. Since it has a lone pair, it suffers Lone pair-bond pair (LP-BP) repulsion, and LP-BP repulsion always leads to a decrease in bond angle. However, the bond angle after LP-BP repulsion is indeed greater than the bond angles of Drago molecules. So, being a Drago molecule, PH3 has exceptionally small bond angle.
10 months ago
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