Question

What is the formal charge on each atom in the tetrahydridoborate ion?

Answer 1

Well, we normally represent sodium borohydride as `Na^+BH_4^-`...

Explanation:

...and since this is clearly NEUTRAL...we split the salt up in to `Na^+` and `BH_4^(-)` ions....

...for the tetrahedral borohydride ion....there are `4xxB-H` bonds....each hydrogen atom is associated with ONE electron, and is therefore neutral. On the other hand, the boron atom get 4 electrons from the `B-H` bonds, and there are two inner core electrons....six electrons in total. Since for boron, `Z=5`, given 5 nucular charges, and SIX electronic charges....the boron atom has a formal negative charge.

Borohydrides ACT as hydride transfer reagents in their reaction chemistry....i.e. `BH_4^(-)-=BH_3 + H^-`...they tend to be a bit milder reagents than aluminum hydrides...salts of `AlH_4^(-)`...