GS-2 Physical Sciences Question 38

[student]

38. Which of the following values do you think is closest to the bond angle in white phosphorus P4 molecules?

the answer is 60 degrees

but the way i thought about it was that each phosphorous is sp3 hybridized and that each angle is therefore about 109.5. what is wrong with my logic?

[MCATsFriedMyBrain]

Each P has a lone pair of electrons. So each P has 3 sp2 orbitals and one p orbital. Applying VESPR, you get a trigonal pyrimidal configuration with 60 degrees angles between the bonds.

random...I just found 30 dollars in my MCAT book while I was referencing VESPR models. Weird.

[admin]

Hey! That was my $30 !

You described P correctly. I would only add the following: there was really only one possible answer for Q36 (D) and if you look at that drawing of P4 carefully, you'll note that every side of the pyramid is clearly an equilateral triangle, then each angle must be 60 degrees.

In other words, your options were: (1) you memorized info about P4 (bad idea); (2) you worked it out based on VESPR models (very good); or (3) you went with the flow of the passage and the questions which permitted you to deduce the right answer (best idea since it allows you to answer soooooo many more questions correctly).

[admin]

Good question. Is an sp2 hybrid always 120 degrees? Nope. In fact, in another classic MCAT type question, you can be tested on your understanding that trigonal planar is indeed 120 but trigonal pyramid (like NH3 because of the lone pair electrons causing repulsion of the bonds), the bond angleis more like 109 degrees despite the sp2 hybridization.

What is the bond angle for sp3 hybridized carbon in propane. You would be correct to suggest the tetrahydral angle 109.5 degrees. But isn't cyclopropane also sp3 hybridized? Yep, and its bond angles are forced to be 60 degrees because it forms a triangle.

Ref: www.wpbschoolhouse.btinternet.co.uk/page06/molecule_shapes.htm

Of course, you already know that the maximum number of degrees in a triangle is 180 degrees (examples: the 30-60-90 triangle, the 45-45-90 triangle). So if there are 3 P's in an equilateral triangle (each of the 3 angles must be the same) then the angle must be 60 degrees each.

[admin]

Yes, NH3 is sp3 hybridized. I was writing too fast!

[mytramta1691]

Can you please explain for me why the N in NH3 has 3 bonds and 1 lone pair and have sp3 , but each P in P4 also have 3 bonds and 1 lone pair but have sp2 ?
I kind of understand the NH3 because 3 bonds + 1pair e- = 4 = sp3. Why is it not the same for P4

Thanks a lot