* Atoms are the basic building blocks of matter. They consist of a nucleus (protons and neutrons) and electrons. A single phosphorus atom would be represented as "P".

* Molecules are formed when two or more atoms bond together. For example, a molecule of phosphorus (P4) consists of four phosphorus atoms bonded together.

* Ions are atoms or molecules that have gained or lost electrons, giving them a net electrical charge. P3- is a phosphorus atom that has gained three electrons, giving it a negative charge.

Therefore, P3- represents a phosphorus ion with a charge of -3.