This element comes in violet, black, red, and white allotropes.
An iodide of a methyl group attached an atom of this element, which is attached to three phenyl groups, is used to form the ylide for the Wittig reaction. The simplest example of a divalent compound breaking the octet rule is this element's pentachloride. This element comes in violet, black, red, and white allotropes. Found just below nitrogen on the periodic table, for 10 points, identify this element, number 15 with symbol P.
An iodide of a methyl group attached an atom of this element, which is attached to three phenyl groups, is used to form the ylide for the Wittig reaction.
{phosphorus} [accept {just P} before mention]