A phosphorus atom has 15 electrons, with 2 in the K shell, 8 in the L shell and 5 in the M shell.
Since it has 5 outer electrons, phosphorus is an element of group 15, like nitrogen. Phosphorus is not found as the element in nature, and it is extracted from phosphate rock, which contains calcium phosphate.
Phosphorus has several allotropes, including white phosphorus and red phosphorus.
White phosphorus is made of \mathrm{P_4} molecules, in which 4 atoms are joined in the shape of a regular tetrahedron. It is highly poisonous and catches fire by itself in air, so it is stored under water.
Red phosphorus is a reddish-brown powder in which many atoms are joined together. It has low toxicity and does not catch fire by itself in air.
The head of a match is coated with substances such as potassium chlorate and sulfur, and the side of the box with substances such as red phosphorus and powdered glass.
When a match is struck on the side of the box, the heat of friction makes a little of the red phosphorus catch fire. This heat makes the potassium chlorate in the head decompose and give off oxygen, and the sulfur and the other substances start to burn.
Because the red phosphorus and the potassium chlorate are kept apart, one on the box and the other on the head, a match is hard to light unless it is struck on the side of the box.
When phosphorus burns, it gives off white smoke and forms tetraphosphorus decaoxide, \mathrm{P_4O_{10}}, which is also called phosphorus(V) oxide.
4\mathrm{P} + 5\mathrm{O_2} \longrightarrow \mathrm{P_4O_{10}}
Tetraphosphorus decaoxide absorbs water very strongly, so it is used as a drying agent. When water is added and the mixture is heated, phosphoric acid forms.
\mathrm{P_4O_{10}} + 6\mathrm{H_2O} \longrightarrow 4\mathrm{H_3PO_4}
Phosphorus is also an element essential to the bodies of living things.
Bones and teeth are made mainly of a kind of calcium phosphate, and DNA, which makes up genes, has phosphate groups lined up along it. ATP, which is used for moving muscles and for other work in the body, is also a substance with three phosphate groups.
Plants need phosphorus too, so phosphorus, together with nitrogen and potassium, is one of the three main nutrients in fertilizers.