Phosphorus
Phosphorus P is in the main subgroup of group V, period 3, outer shell 3s²3p³; oxidation states –3, 0, +3, +5, in nature mainly +5. It does not occur free: it is found as phosphorite Ca₃(PO₄)₂ and apatites (e.g. fluorapatite Ca₅(PO₄)₃F), and in bones, teeth, proteins and nucleic acids. In industry it is made in an electric furnace without air by heating phosphorite with sand and coke: Ca₃(PO₄)₂ + 5C + 3SiO₂ → 3CaSiO₃ + 2P + 5CO. White phosphorus P₄ is molecular, insoluble in water, soluble in CS₂, glows in the dark, ignites spontaneously in air, is very poisonous and is stored under water; it melts at about 44 °C. Red phosphorus has a polymeric structure, is non-poisonous and does not dissolve in CS₂; it forms when white phosphorus is heated without air. Chemically phosphorus combines with oxygen, halogens, sulfur and metals: 4P + 5O₂ → 2P₂O₅, 2P + 3Cl₂ → 2PCl₃, 2P + 3Ca → Ca₃P₂. The striking strip on the side of a matchbox contains red phosphorus and the match head contains an oxidizer (KClO₃): 6P + 5KClO₃ → 5KCl + 3P₂O₅. The main use of phosphorus is for phosphoric acid and fertilizers. Only specialists handle white phosphorus; matches are not a toy.
“Phosphorus cycle”: draw the circle rock → soil → plant → animal → human → waste → soil. Add phosphate fertilizer, phosphorus in bones and teeth, and ATP. Write 3 rules for keeping matches safely. No match is struck.