HW 4.19 (c)

(Polar molecules, Non-polar molecules, etc.)

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Jacob Afable 3J
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Joined: Fri Sep 25, 2015 3:00 am

HW 4.19 (c)

Postby Jacob Afable 3J » Sun Oct 25, 2015 6:59 pm

For BH2-, could someone please explain to me how the solution manual got a bond angle that is slightly less than 120 degrees?

Peiliang Zhou 3K
Posts: 21
Joined: Fri Sep 25, 2015 3:00 am

Re: HW 4.19 (c)

Postby Peiliang Zhou 3K » Sun Oct 25, 2015 7:11 pm

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!

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