Question

Question #c9ba9

Answer 1

`8` electrons.

Explanation:

The number of valence electrons can be found by determining the oxidation number.

In the case of `PCl_5`, the oxidation number of phosphorus can be found from the oxidation number of chloride.
Chlorine is a halogen, which means it needs to gain one electron to achieve noble gas configuration—therefore, its oxidation number in `PCl_5` is `"-1"`.

The oxidation state of all `5` chloride ions add up to be `"-5"`.

Because this is a neutral compound (with no charge), the oxidation number of phosphorus has to be `"+5"`.

From this, we can infer that phosphorus lost five electrons.

Then, just jump back five spaces on the periodic table:

The number of valence electrons `P^("5+")` has is the same as neon: `8`.

Alternatively, we can look at the Bohr model of phosphorus.

Once all `5` electrons have been taken away from the valence shell, the previous shell becomes the valence shell.
That shell has `8` electrons.