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HW 4.19 (c)

Posted: Sun Oct 25, 2015 6:59 pm
by Jacob Afable 3J
For BH2-, could someone please explain to me how the solution manual got a bond angle that is slightly less than 120 degrees?

Re: HW 4.19 (c)

Posted: Sun Oct 25, 2015 7:11 pm
by Peiliang Zhou 3K
The total number of electrons of BH2- is 6e- (3e- in B + 2*1e- in H + 1e-). After you draw the lewis structure, you should have a Boron as the central atom with the two hydrogens bonded with single bonds and a lone electron pair on the central B atom. It should look like this:

. .
/ \

Because the electron density geometry is trigonal planar (and the molecular shape is bent), the bond angles should be 120 degrees each. However, since a lone electron pair has a high repulsion, it pushes the two single bonds downwards and more towards each other, making the bond angle slightly less than what it should be (120 degrees).

Hope this helped!