Give reasons for the following
(a) `PF_(5)` is know but `NF_(3)` is not
(b) `H_(2)O` is a good solvent
(c ) `BF_(3)` is non- polar but planar
(e) Carbon -oxygen `(C-O)` bond lengths are equal in `Na_(2)CO_(3)`
(f) `MgF_(2)` is more soluble in water than `MgCI_(2)` .

Explanation

### Step-by-Step Solution

#### (a) Why is `PF5` known but `NF5` is not?

1. **Valence Electrons**: Both nitrogen (N) and phosphorus (P) have 5 valence electrons.
2. **Covalency**: Phosphorus can exhibit a maximum covalency of 5 due to the availability of d orbitals, allowing it to form five bonds in `PF5`.
3. **Nitrogen's Limitation**: Nitrogen, on the other hand, lacks d orbitals and typically exhibits a maximum covalency of 3, which allows it to form `NF3` but not `NF5`.
4. **Conclusion**: Therefore, `PF5` exists because phosphorus can expand its valence shell, while `NF5` does not exist due to nitrogen's inability to do so.
...