Phosphorus
Atomic Data
| Atomic Number | 15 |
| Symbol | P |
| Atomic Weight | 30.974 u |
| Density (STP) | 1.823 g/cm³ |
| Melting Point | 44.15 °C (317.3 K) |
| Boiling Point | 280.5 °C (553.65 K) |
| Electronegativity | 2.19 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p3 |
| Oxidation States | −3, +1, +3, +5 |
| Phase at STP | Solid |
| Category | Reactive Nonmetal |
| Period / Group | 3 / 15 |
| CAS Number | 7723-14-0 |
Electron Configuration
[Ne] 3s2 3p3
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 3 | 15 |
| Total | 15 | 15 | ||
Isotopes of Phosphorus
Phosphorus is monoisotopic — ³¹P is its only naturally occurring stable isotope, accounting for 100% of all natural Phosphorus.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Phosphorus-31 | ³¹P | 15 | 16 | 100 | Stable |
Abundance & Occurrence
Phosphorus is present in Earth's crust at approximately 1050 ppm by mass and at approximately 7 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- White phosphorus — extreme fire hazard: White phosphorus ignites spontaneously in air above ~30 °C, burning with a dense white smoke; it must be stored under water and handled in an inert atmosphere or under water at all times.
- White phosphorus toxicity: White phosphorus is acutely toxic; ingestion of as little as 50 mg can be fatal, causing severe liver damage. Skin contact causes deep, slow-healing chemical burns that reignite on exposure to air.
- Red phosphorus — fire hazard: Red phosphorus is far less hazardous than white, but is still a fire risk; friction or impact with oxidisers (e.g. chlorates) can cause ignition — the basis of safety match heads.
- Phosphine gas: Phosphine (PH3), produced when phosphides contact water, is extremely toxic and pyrophoric; it is used as a fumigant and requires strict containment and respiratory protection.
- Environmental persistence: Organophosphate compounds derived from phosphorus chemistry include nerve agents and pesticides; improper disposal can cause serious environmental and ecotoxicological harm.
Phosphorus in the Real World
Real-World Uses
- Fertilisers — Phosphate rock is processed into superphosphate, triple superphosphate, and ammonium phosphate fertilisers that replenish soil phosphorus depleted by crop harvesting, supporting global food production.
- Detergents — Sodium tripolyphosphate (STPP) softens water by sequestering calcium and magnesium ions, improving cleaning efficiency in dishwasher powders and laundry detergents; its use has been restricted in some regions due to aquatic eutrophication concerns.
- Matches and incendiaries — Red phosphorus is the friction-sensitive compound on safety match strike surfaces; white phosphorus was historically used in military smoke grenades and incendiary munitions.
- DNA, RNA, and biochemistry — Phosphate groups form the backbone of every DNA and RNA molecule and are central to energy transfer via ATP (adenosine triphosphate), making phosphorus indispensable to all living cells.
- Steel and metallurgy — Small amounts of phosphorus are added to steel to improve machinability; it is also used in phosphor bronze (Cu-Sn-P) for springs, bearings, and electrical connectors.
- Flame retardants — Organophosphorus compounds are used as halogen-free flame retardants in electronics, textiles, and plastics, replacing brominated compounds due to lower environmental toxicity concerns.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is phosphorus used for?
Phosphorus is used primarily in fertilisers (as phosphate rock derivatives) that sustain modern agriculture. It also appears in detergents (as sodium tripolyphosphate), safety matches (red phosphorus on the strike surface), food additives (phosphoric acid in cola drinks), pharmaceutical drugs, flame retardants, and in the chemical form of DNA and ATP, making it biologically indispensable.
Why is phosphorus essential for life?
Phosphorus is fundamental to all life. Phosphate groups form the backbone of DNA and RNA, linking nucleotide units together. ATP (adenosine triphosphate) stores and transfers biochemical energy using phosphate bonds. Phospholipids form the cell membrane bilayer. Calcium phosphate minerals (hydroxyapatite) make up bones and teeth. No substitute for phosphorus has been found in any living system, making it uniquely irreplaceable in biology.
What are the different forms of phosphorus?
Phosphorus has several important allotropes. White phosphorus is a waxy, very reactive, and highly toxic solid that ignites spontaneously in air at about 30°C. Red phosphorus is a more stable amorphous polymer, less reactive and far less toxic, used on safety match boxes. Black phosphorus is the most thermodynamically stable form, a graphite-like layered semiconductor with promising electronic properties. Violet phosphorus is a fourth crystalline form of industrial interest.
Is phosphorus running out?
Phosphate rock, the primary source of agricultural phosphorus, is a non-renewable resource. There is no phosphorus cycle equivalent that can substitute for mining phosphate rock to replenish soil. Current accessible reserves are concentrated in Morocco (about 70% of world reserves), with significant deposits in China and other countries. Concerns about 'peak phosphorus' have led to growing interest in phosphorus recovery from wastewater, food waste, and animal manure to close the nutrient cycle.