Which of the following compounds contains phosphorus (P) with an oxidation number of -3?

• A. PH3
• B. Na3P
• C. H3PO4
• D. PCl3

Answer: (B)

Verified step by step guidance

1. Recall the definition of oxidation number: it is the hypothetical charge an atom would have if all bonds to atoms of different elements were 100% ionic.
2. Identify the oxidation number of phosphorus (P) in each compound by using the known oxidation states of other atoms and the fact that the sum of oxidation numbers in a neutral compound is zero.
3. For PH3: Hydrogen (H) usually has an oxidation number of +1. Since there are 3 hydrogens, total +3, phosphorus must be -3 to balance the molecule.
4. For Na3P: Sodium (Na) has an oxidation number of +1. With 3 sodium atoms, total +3, phosphorus must be -3 to balance the compound.
5. For H3PO4 and PCl3: Use the known oxidation states of oxygen (-2) and chlorine (-1) respectively to calculate phosphorus oxidation number, which will not be -3 in these cases.