SolutionsEight Electrons
1. Shared and unshared pairs
State how many shared and unshared pairs surround the central atom in BH3, CH4, NH3 and H2O. Which one does not obey the octet rule?
| Molecule | Shared pairs | Unshared pairs | Electrons around |
|---|---|---|---|
| BH3 | 3 | 0 | 6 |
| CH4 | 4 | 0 | 8 |
| NH3 | 3 | 1 | 8 |
| H2O | 2 | 2 | 8 |
The boron of BH3 has only six electrons: it does not obey the octet rule.
2. Giving and accepting
Which ions do magnesium and fluorine form? Write the formula of the compound they form with each other.
Mg2+ (it gives up two electrons) and F− (it accepts one). Two fluorides are needed for each magnesium: MgF2.