Phosphorus at a glance
- Atomic number
- 15
- Symbol
- P
- Atomic mass
- 30.974 u
- Category
- Reactive nonmetal
- Group / period
- 15 / 3
- Electron configuration
- [Ne] 3s 2 3p 3
- State at 25 °C
- Solid
- Melting point
- 44.15 °C
- Boiling point
- 280.5 °C
- Density
- 1.82 g/cm³
- Electronegativity
- 2.19
- 1st ionization energy
- 1012 kJ/mol
- Oxidation states
- +5, +3, −3
- Discovered
- 1669
Atomic structure
A neutral phosphorus atom has 15 protons and 15 electrons, and its most common isotope carries about 16 neutrons. One atom weighs about 5.14 × 10⁻²⁶ kg, so a mole of phosphorus (6.022 × 10²³ atoms) weighs 30.974 g. That nucleus holds 1.07 neutrons for every proton. The electrons fill 3 shells (K 2, L 8, M 5), which gives the configuration [Ne] 3s 2 3p 3. With 5 electrons in its outer shell, it tends to gain or share electrons to complete its outer shell. It has only one stable isotope, P-31. No other element has a stable isotope with mass number 31, so P-31 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) phosphorus is a solid. It melts at 44.15 °C and boils at 280.5 °C, so it stays liquid across a 236.3-degree range. It stays solid in your hand (body temperature) but would melt in boiling water. Its density is 1.82 g/cm³, 1.82 times as dense as water. Its atoms have a radius of about 100 pm (Silicon next door is 110 pm). In the solid, its atoms pack into an orthorhombic lattice.
Chemical behaviour
Its electronegativity is 2.19 on the Pauling scale. Pulling off its outermost electron takes 1012 kJ/mol, less than Nitrogen above it (1402 kJ/mol). A bond between phosphorus and chlorine would be polar covalent, with an electronegativity difference of 0.97. It releases 72 kJ/mol when it gains an extra electron. Its common oxidation states are +5, +3, −3, so in compounds you will usually meet it as P⁵⁺, P³⁺ and P³⁻. Group 15 also holds Nitrogen, Arsenic, Antimony, Bismuth and Moscovium, which share its outer-shell pattern. Its Lewis symbol is P surrounded by 5 dots. Handle with care: as a pure element it is toxic and flammable or reacts with water.
Where it comes from
It makes up 1050 mg/kg of Earth's crust, the 11th most abundant element there. In the human body it accounts for about 1 % of body mass, making it one of the main building blocks of life. It was first identified in 1669, making it the 13th element to be discovered (counting the 11 known since antiquity together). In a flame test its compounds colour the flame pale blue-green.