Phosphorus
Atomic Data
| Atomic Number | 15 |
| Symbol | P |
| Atomic Weight | 30.974 u |
| Density (STP) | 1.823 g/cm³ |
| Melting Point | 44.15 °C (317.30 K) |
| Boiling Point | 280.50 °C (553.65 K) |
| Electronegativity | 2.19 (Pauling) |
| Electron Config. | 1s² 2s² 2p&sup6; 3s² 3p³ |
| Oxidation States | +5, +3, +1, −3 |
| Phase at STP | Solid |
| Category | Reactive nonmetal |
| Period / Group | 3 / 15 |
| CAS Number | 7723-14-0 |
Electron Configuration
1s2 2s2 2p6 3s2 3p3
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| 2p | 6 | 10 | ||
| M | 3 | 3s | 2 | 12 |
| 3p | 3 | 15 | ||
| Total | 15 | 15 | ||
Isotopes of Phosphorus
Phosphorus has only one naturally occurring isotope, ³¹P, which accounts for 100% of all natural phosphorus and is perfectly stable. This makes phosphorus a monoisotopic element — one of just 22 such elements in the periodic table.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Phosphorus-31 | ³¹P | 15 | 16 | 100% | Stable |
Abundance & Occurrence
Phosphorus is the eleventh most abundant element in Earth’s crust, found at roughly 1050 parts per million. It never occurs as free elemental phosphorus in nature because it reacts readily with oxygen; instead it is found as phosphate minerals, the most economically important of which is apatite. In the universe phosphorus is relatively scarce at around 7 ppm by mass, produced through nucleosynthesis in massive stars.
EARTH'S CRUST ABUNDANCE (BY MASS, SELECTED ELEMENTS)
HUMAN BODY COMPOSITION (BY MASS)
Discovery & History
Safety & Handling
- White phosphorus is highly toxic: the estimated lethal dose for humans is around 1 mg per kilogram of body weight. Ingestion or prolonged skin contact can cause severe liver and kidney damage. Historically it caused “phossy jaw,” a disfiguring necrosis of the jaw, among match workers.
- Spontaneous ignition: white phosphorus ignites in air at approximately 30 °C and burns intensely, producing dense white phosphorus pentoxide smoke. It must be stored submerged in water and kept away from all heat sources and oxidising agents.
- Red phosphorus is considerably safer — stable in air at room temperature — but can ignite by friction, shock, or temperatures above 240 °C. Avoid mixing with oxidisers such as chlorates or peroxides, which can produce explosively sensitive mixtures.
- Phosphine gas (PH₃) is a highly toxic and flammable gas produced from some phosphorus reactions and decomposing organic matter. OSHA limits occupational exposure to 0.3 ppm over an 8-hour shift.
- Fire response: fires involving phosphorus should be fought with dry sand or carbon dioxide extinguishers. Water can scatter burning particles. Burning phosphorus produces toxic phosphorus pentoxide fumes; evacuation and respiratory protection are required.
- Environmental hazard: phosphate runoff from agricultural fertilisers causes eutrophication in rivers and lakes, triggering algal blooms that deplete dissolved oxygen and devastate aquatic ecosystems.
Phosphorus in the Real World
Real-World Uses
- Fertilisers and agriculture — The largest use of phosphorus worldwide. Mined phosphate rock is converted into superphosphate and diammonium phosphate fertilisers that replenish soil phosphate depleted by crops. Without phosphate fertilisers, modern agricultural yields could not sustain the current global population.
- Detergents — Sodium tripolyphosphate (STPP) acts as a water softener in detergents, preventing calcium and magnesium ions from interfering with soap performance. Its use has been restricted in many countries due to the eutrophication it causes when phosphate-laden wastewater reaches waterways.
- Match heads — Red phosphorus is coated on the striking surface of safety matches. When the match head — containing an oxidising agent — is dragged across the surface, friction ignites the red phosphorus and lights the match. This replaced the earlier, highly toxic white-phosphorus match formulations.
- Flame retardants — Organophosphorus compounds are widely incorporated into plastics, textiles, and printed circuit boards as flame retardants. They interrupt combustion by releasing phosphoric acid in the flame, which forms a protective char layer that limits oxygen access to the burning material.
- DNA, RNA, and ATP — Phosphate groups form the negatively charged sugar-phosphate backbone of every DNA and RNA molecule. Adenosine triphosphate (ATP), the universal energy currency of living cells, stores and releases chemical energy through the breaking and reforming of phosphate bonds in metabolic reactions across all known life.
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 and agriculture, where it replaces soil phosphate depleted by crops. It is also used in detergents, match heads, flame retardants, and is indispensable in biology as the backbone of DNA and RNA and the central atom in ATP, the molecule that powers cellular energy.
Is phosphorus dangerous?
White phosphorus is highly toxic and spontaneously ignites in air at around 30 °C, making it extremely hazardous. Red phosphorus is far more stable and is used safely in match heads and flame retardants. Elemental phosphorus should always be handled away from air, heat, and ignition sources. Phosphorus compounds such as phosphine gas are also acutely toxic.
Who discovered phosphorus?
Phosphorus was first isolated in 1669 by German alchemist Hennig Brand, who distilled it from large quantities of urine in search of the Philosopher’s Stone. It was the first element to be discovered by a known individual and the first isolated through a documented chemical process.
Why is phosphorus important to life?
Phosphorus is essential to all living things. Phosphate groups form the sugar-phosphate backbone of DNA and RNA, store and transfer energy as adenosine triphosphate (ATP), and are a key structural component of cell membranes as phospholipids. Bones and teeth are largely made of calcium phosphate. Without phosphorus, life as we know it could not exist.
What are the different forms of phosphorus?
Phosphorus exists in several allotropic forms. White phosphorus is a waxy, toxic solid that glows faintly in air and ignites spontaneously at around 30 °C. Red phosphorus is a polymeric solid that is stable in air and used commercially. Black phosphorus is the most stable allotrope, with a layered structure similar to graphite, and is of growing research interest in electronics and semiconductors.