Phosphorus is a reactive group-15 nonmetal with several allotropes: white (waxy, toxic, pyrophoric), red (more stable, polymeric), and black (layered, semiconducting).
White (or yellow) P: molecular solid of P4 tetrahedra; soft, waxy, toxic, pyrophoric (ignites in air), stored under water.
Red P: polymeric network derived from P4; more stable, non-pyrophoric; used in safety matches.
Black P: layered, puckered sheets (phosphorene monolayers); semiconducting with anisotropic properties.
Ground-state configuration: \([Ne]3s^2\,3p^3\) with three unpaired p-electrons.
Common oxidation states: −3 (e.g., PH3), +3 (e.g., PCl3, H3PO3), and +5 (e.g., PCl5, H3PO4, PO43−).
Each P in P4 is sp3-like and forms three P–P bonds in a strained tetrahedron (bond angles ≈ 60°). Angle strain and lone pairs make P4 highly reactive and susceptible to oxidation and halogenation.
PCl3: from direct chlorination of white/red P under controlled conditions: \(\mathrm{P_4 + 6\,Cl_2 \rightarrow 4\,PCl_3}\). It is a Lewis base/ligand and chlorinating agent.
PCl5: chlorination of PCl3: \(\mathrm{PCl_3 + Cl_2 \rightarrow PCl_5}\). Acts as a chlorinating and dehydrating agent; in solution may ionize to [PCl4]+ and [PCl6]−.
Important oxyacids include:
General neutralization example: \(\mathrm{H_3PO_4 + 3\,NaOH \rightarrow Na_3PO_4 + 3\,H_2O}\).
Wet process: Attack phosphate rock (fluoroapatite) with sulfuric acid to form phosphoric acid and gypsum:
\(\mathrm{Ca_5(PO_4)_3F + 5\,H_2SO_4 + 10\,H_2O \rightarrow 3\,H_3PO_4 + 5\,CaSO_4\cdot 2H_2O + HF}\)
Thermal process: Burn elemental P to P4O10 then hydrate to H3PO4, giving higher purity acid.
Phosphine is a toxic, colorless gas; weakly basic and a reducing agent. It may ignite spontaneously when contaminated with P2H4. Typical lab preparation (from phosphides): \(\mathrm{Ca_3P_2 + 6\,H_2O \rightarrow 3\,Ca(OH)_2 + 2\,PH_3}\).
Phosphorus is essential in ATP/ADP energy transfer, nucleic acids (DNA/RNA), and phospholipid membranes. ATP hydrolysis releases energy:
\(\mathrm{ATP + H_2O \rightarrow ADP + P_i + \Delta G}\)
Dietary phosphates are vital for bone and teeth (as hydroxyapatite).
Excess phosphate from fertilizers/detergents can cause eutrophication: algal blooms reduce dissolved oxygen, harming aquatic life. Best practices include controlled application, buffer zones, and wastewater phosphate removal.
White P is pyrophoric and toxic. Modern safety matches separate reagents: the striking surface contains red P and powdered glass; the match head contains oxidizer (e.g., KClO3) and binder. Friction converts some red P to white P locally, initiating ignition—safer and avoids chronic toxicity associated with white P.