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 $\ce{P + Cl -&gt; PCl5}$. Is $\ce{P -&gt; P4}$ or just $\ce{P}$? Also, would $\ce{Cl}$ be diatomic too ($\ce{Cl2}$) in this equation?
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