Question

What is the structural formula for `PH_3`?

Answer 1

The structural formula is how the molecule is actually drawn out, but this is a fairly simple molecule.

In the case of `"PH"_3`, you know that hydrogen only has one electron, and so it can only make a maximum of one single bond (most of the time).

Therefore, phosphorus (`"P"`) must be the central atom, and you just draw hydrogen atoms around it. Phosphorus is right below nitrogen, so it has the same number of valence electrons as nitrogen.

As a result, `"PH"_3` (phosphorus trihydride) and `"NH"_3` (ammonia) are isoelectronic (same number of electrons, or similar electronic structure), and they have the same molecular geometry as well:

(bond angles approximately to scale.)