How many total electrons are in p^-3? A. 18 B. 34 C. 15 D. 12?

"Phosphide ion"......

Your teacher wants you to say that there are three more electrons than the neutral atom has, so that is 15+3 or 18.

But the problem is with "phosphide ion" and it's -3 charge. Phosphorous is some compounds has an oxidation state of -3, but that doesn't mean that it has an actual charge of -3. Phosphorous has a relatively low electron affinity, meaning that there is little tendency for a neutral phosphorous atom to gain an electron to form P^-. But what about gaining a second electron? There is little chance that the negative P^- ion will gain a second electron because of the repulsion between like charges, and almost no chance that it would form an ion with a -3 charge.

Phosphorous does have an oxidation state of -3, but the oxidation state is not the same as an actual ionic charge. The oxidation state of an element is the charge on an atom if the bonds are 100% ionic, but of course, we know that there are no 100% ionic bonds. If we coupled phosphorous with the most electropositive metal, the bond would have about 38% ionic character (or about 62% covalent character).

Therefore, despite the "answer" of "18 electrons" (which satisfies your teacher) it doesn't actually reflect reality.

atomic number of phosphorus is 15, so when 3 extra electrons are added, there are 18.

Atomic number of phosphorus is 15.

An atom of P contains 15 protons and 15 electrons.

P^3-has 3 more electrons to give a total of 18 electrons.