Phosphorus
Phosphorus is a chemical element with symbol P and atomic number 15. As an element, phosphorus exists in two major forms—white phosphorus and red phosphorus—but due to its high reactivity, phosphorus is never found as a free element on Earth. Instead phosphorus-containing minerals are almost always present in their maximally oxidised state, as inorganic phosphate rocks.
At a glance
- Atomic number
- 15
- Atomic mass
- 30.973762 u
- Phase
- Solid
- Density
- 1.823 g/cm³
- Melting point
- Unavailable
- Boiling point
- Unavailable
- First ionization energy
- 1,011.8 kJ/mol
- Electronegativity
- 2.19
- Electron affinity
- 72.037 kJ/mol
- Molar heat capacity
- 23.824 J/(mol·K)
- Period
- 3
- Appearance
- colourless, waxy white, yellow, scarlet, red, violet, black
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Phosphorus’s orbitals use a hydrogen-like approximation. Every orbital of each visible subshell is shown: in Shapes as the surface holding half its probability, with light and dark shades for opposite signs of the wavefunction; in Cloud as dots where its electrons are likely to be. Sizes are stepped by shell, like the Shell model.
10 is the [Ne] core total across two shells.
Preparing the orbital view…
15 protons16 neutrons
Phosphorus-31: its most common form in nature, or its longest-lived if it has none. The nucleus is drawn far too big: it’s about 100,000 times smaller than the atom.
Turn up the heat
A step of 1 K is the same size as 1 °C, so the two scales are always 273.15 apart. Fahrenheit is different: its degrees are smaller and it starts elsewhere, so water freezes at 273.15 K, 0 °C and 32 °F.25 °C · 77 °FSolid
At this temperature, phosphorus is a solid.
Values for white phosphorus. Phosphorus has several forms, and red and black phosphorus behave differently when heated.
At normal atmospheric pressure (1 atm). The temperature scale is logarithmic. The particle box is schematic: real samples hold vastly more atoms, and spacing and speeds are not to scale.
Pull the electrons off
PNeutral atom · no charge
A neutral phosphorus atom. Removing its first electron takes 1,012 kJ/mol.
Phosphorus’s outer shell holds 5 electrons. Electron 6 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 3.4× as much energy as the one before. Each dashed line on the chart marks another jump, into a shell even closer to the nucleus.
What does this tell us?
- The jumps let you count electrons. Between the dashed lines there are 5, then 8, then 2 bars. Read backwards, that’s 2, 8, 5: the same electrons per shell as the shell model above. Jumps like these are part of the evidence that shells exist.
- The 5 electrons before the first big jump are phosphorus’s valence electrons, the ones it uses in chemical reactions. That’s why phosphorus sits in group 15: every element in the group has the same number of outer electrons, so they react in similar ways.
- Removing electrons from phosphorus is expensive from the start (1,012 kJ/mol for the first). So in reactions phosphorus tends to gain or share electrons rather than lose them.
- Within a shell the steps rise gently. Each electron removed leaves fewer electrons pushing the others away, while the nucleus pulls just as hard, so the next one is held a little more tightly.
Show the values as a table
| Electron | Shell | Energy (kJ/mol) | × previous | Value |
|---|---|---|---|---|
| 1 | 3 | 1,012 | – | Measured |
| 2 | 3 | 1,908 | 1.9 | Estimated |
| 3 | 3 | 2,914 | 1.5 | Measured |
| 4 | 3 | 4,964 | 1.7 | Measured |
| 5 | 3 | 6,274 | 1.3 | Measured |
| 6 | 2 | 21,268 | 3.4 | Estimated |
| 7 | 2 | 25,431 | 1.2 | Estimated |
| 8 | 2 | 29,872 | 1.2 | Estimated |
| 9 | 2 | 35,922 | 1.2 | Estimated |
| 10 | 2 | 40,948 | 1.1 | Estimated |
| 11 | 2 | 46,259 | 1.1 | Estimated |
| 12 | 2 | 54,092 | 1.2 | Estimated |
| 13 | 2 | 59,024 | 1.1 | Calculated |
| 14 | 1 | 271,790 | 4.6 | Calculated |
| 15 | 1 | 296,195 | 1.1 | Calculated |
Successive ionization energies of gas-phase atoms and ions from the NIST Atomic Spectra Database. Solid bars are measured; hatched bars were estimated from nearby measurements or calculated from quantum theory, as is usual for highly charged ions. Electrons are taken from the outermost shell first, a simplification: electrons in different subshells of the same shell also differ in energy. On the log scale each gridline is 10 times the one below.
Same atoms, three different materials
White, red and black phosphorus are all pure phosphorus. The only difference is how the atoms are joined up, and that changes everything: one glows in the dark and bursts into flame on a warm day, one sits safely on a matchbox, and one is as calm as graphite.
White phosphorus
- Built from
- Separate P₄ molecules: four atoms in a pyramid
- Bond angle
- 60°, squeezed far tighter than phosphorus likes
- Looks like
- A soft, waxy, pale yellow solid
- In air
- Glows green in the dark; catches fire at about 30 °C
- Made by
- Condensing phosphorus vapour
- Toxic?
- Very. It’s stored under water
- Density
- 1.82 g/cm³
Each P₄ pyramid has its bonds squeezed to 60°, like bent springs. They snap open easily, which makes white phosphorus fiercely reactive.
Warm them up
- White phosphorus
- Glows faintly (try lights off)
- Red phosphorus
- Nothing happens
- Black phosphorus
- Nothing happens
Phosphorus in history and today
- Found in urine
In 1669 Hennig Brand boiled down buckets of urine hoping to make gold. He got a waxy solid that glowed in the dark: white phosphorus. Its name is Greek for “light-bearer”.
- Matches
The first matches used white phosphorus, which poisoned the factory workers who made them. Safety matches keep red phosphorus on the side of the box instead: the strike turns a tiny amount into something that lights.
- One atom thick
Peel black phosphorus down to a single crinkled sheet and you get phosphorene, first made in 2014. Like graphene, it’s being studied for tiny, fast electronics.
Atoms are drawn smaller than their true size so the structures are clear, and use the standard model colour for phosphorus, orange. Red phosphorus is really a tangle of chains; one tidy chain is shown. Never try this at home: white phosphorus is dangerous.
How does it compare?
Density
1.82 g/cm³19th lowest of 99
Melting point
317 K17th lowest of 96
Boiling point
554 K14th lowest of 97
Electronegativity
2.1923rd highest of 96
First ionization energy
1,012 kJ/mol14th highest of 99
Elements 1–99 from the reference data; heavier elements are left out because only a few atoms have ever been made, so their bulk properties are predictions. First ionization energies come from NIST. Density uses a logarithmic scale, because gases are around a thousand times less dense than solids; gas densities are at 0 °C and 1 atm. Melting and boiling points are at normal pressure. Everyday temperatures and light sources in the pictures are approximate. Hover over a dot to see the element, or click it to compare.
Keep exploring
Explore the stories, imagery, and reference data in more depth. Properties with no value in the reference are marked unavailable.
- PhosphorusRoyal Society of Chemistry
- PhosphorusWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database