# When is phosphorus a polyatomic molecule?

Phosphorus is listed as a single element in the periodic table; however, I see that it is also a polyatomic molecule as $\ce{P4}$. I am trying to determine the number of atoms of phosphorus in an chemical equation, so I can have a balance equation. So, when would I use the polyatomic version, or the single atomic version if the chemical equation is a word equation, and the subscripts are not listed? For example the word equation is $\ce{P + Cl -> PCl5}$. Is $\ce{P -> P4}$ or just $\ce{P}$? Also, would $\ce{Cl}$ be diatomic too ($\ce{Cl2}$) in this equation?

• If there are many allotropes single atom is generally used. – Mithoron Apr 28 '15 at 23:11
• First make sure you're dealing with elemental phosphorus and not something else with P in it. For example, in agriculture it's common to talk about P levels, P concentration, P determination, etc, but that's never elemental phosphorus but actually phosphate ($\ce{PO4^{3-}}$) salts/acid/minerals/etc. – Molx Apr 29 '15 at 1:17

Phosphorus is most common as white phosphorus (but more stable as red phosphorus) and in this form, it exists as a tetrahedron, or $\ce{P4}$. Chlorine is most stable as a diatomic molecule, as you may have learnt it with the other common gases that exists as diatomic molecules