Phosphorus, a chemical element with symbol P and atomic number 15. Phosphorus is essential for life. Phosphates (compounds containing the phosphate ion, PO4−3) are a component of DNA, RNA, ATP, and also the phospholipids, which form all cell membranes. Demonstrating the link between phosphorus and life, elemental phosphorus was first isolated from human urine, and bone ash was an important early phosphate source.