The electron domain and molecular geometry of nf₃ are
a. octahedral, octahedral
b. tetrahedral, trigonal
c. pyramidal octahedral, square pyramidal
d. tetrahedral, tetrahedral
e. tetrahedral, bent

The electron domain and molecular geometry of NF₃ are option b

Explanation:

First of all, think the total valence electrons of each element.

N : 5

F: 7, but we have 3, so 7.3 = 21

Total: 26 e⁻

Now let's draw a scheme

             F    N    F

                          |

                         F

We have to put 26 e⁻ around the elements, so 6 e⁻ are been used for the bonds N-F, so now, there are 20 e⁻  to add. As the F, have used 1 e⁻ to form the bond, there are 6 more, around. Each fluorine which has 7 e⁻,  shares 1e⁻ with the N, to complete the octet rule.

So, there are still 2e⁻ to add. They will be above the N and they are free. That's why the molecular geometric is trigonal, and the electronic geeometry is tetrahedral because the non-binding electron pair of N

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.