Question

why is bond angle in OF2 is smaller than bond angle in H2O?

Answer 1

It's only a small difference (103 degrees versus 104 degrees in water), and I believe the usual rationalization is that since F is more electronegative than H, the electrons in the O-F bond spend more time away from the O (and close to the F) than the electrons in the O-H bond. That shifts the effective center of the repulsive force between the bonding pairs away from the O, and hence away from each other. So the repulsion between the bonding pairs is slightly less, while the repulsion between the lone pairs on the O is the same -- the result is the angle between the bonds is a little less.

Answer 2

I see, but in my book there is another rationalization too. It has rationalized it by Bent rule and can't understand it :(
it says that more P character in hybrid orbitals the smaller bond angle is. why?!

Answer 3

That's not different, that's just another way of describing the result. To have "more p character" in the hybrid bonds is just another way of saying the bond angle is closer to 90 degrees, which is what it would be if the bonds between the oxygen and the ligand atoms were formed strictly from the oxygen's 2p orbitals (which are 90 degrees apart).

I wouldn't call that an explanation, though. More of a statement of consistency. An *explanation* tells you WHY there is more p character in the hybrid bonds, or equivalently why the bond angle is lower.

Here's an analogy: if you say "I'm broke!" and I ask "Why?" you could give me two answers:

(1) "Because I have no money in the bank!"

(2) "Because a check I wrote a long time ago and forgot about finally cleared."

The first is just a restatement of the problem. I'm broke = I have no money in the bank. The second is an actual explanation, it tells you WHY you have no money in the bank. Your book is offering you (1), a restatement of the observation, while I (and more advanced, or better-written textbooks) would give you (2), an explanation.

What book is this? I like to keep track of deficient reasoning or explanation in chemistry textbooks.

Answer 4

got it, thanks a lot :)
it's my teachers book in the university, "general chemistry molecular approach". it's quite difficult!