A sulfur atom has 16 electrons, with 2 in the K shell, 8 in the L shell and 6 in the M shell.
Since it has 6 outer electrons, sulfur is an element of group 16, like oxygen. Also like oxygen, sulfur can gain 2 electrons to form the doubly charged negative ion \mathrm{S}^{2-}, the sulfide ion.
Near volcanoes, sulfur is found as the element itself, a yellow solid.
Sulfur has several allotropes, including rhombic sulfur, monoclinic sulfur and plastic sulfur.
Rhombic sulfur and monoclinic sulfur are both made of \mathrm{S_8} molecules, in which 8 atoms are joined in a ring. At room temperature, rhombic sulfur is the most stable form, and the sulfur found in nature is rhombic sulfur too. Monoclinic sulfur consists of needle-shaped crystals that form when molten sulfur is cooled in air.
Plastic sulfur forms when sulfur heated to about 250 °C is poured into water and cooled quickly. It is made of molecules in which many atoms are joined in long chains, and it is elastic: it stretches when pulled and goes back to its original shape when let go.
Both monoclinic sulfur and plastic sulfur slowly turn into rhombic sulfur when left at room temperature.
When sulfur is heated in air, it burns with a blue flame to form sulfur dioxide, \mathrm{SO_2}.
\mathrm{S} + \mathrm{O_2} \longrightarrow \mathrm{SO_2}
Sulfur dioxide is a colourless, poisonous gas with a pungent smell. It dissolves in water to form sulfurous acid, which is weakly acidic. Sulfur dioxide formed when the sulfur in coal and oil burns is one of the causes of acid rain.
Sulfuric acid is manufactured by a method called the contact process. First, sulfur is burned to form sulfur dioxide, which is then reacted with oxygen from the air, using vanadium(V) oxide as a catalyst, to form sulfur trioxide, \mathrm{SO_3}.
2\mathrm{SO_2} + \mathrm{O_2} \longrightarrow 2\mathrm{SO_3}
Sulfur trioxide reacts with water to form sulfuric acid.
\mathrm{SO_3} + \mathrm{H_2O} \longrightarrow \mathrm{H_2SO_4}
However, if sulfur trioxide is absorbed directly into water, so much heat is given out that a mist forms, and the sulfur trioxide is hard to absorb. Instead, sulfur trioxide is absorbed into concentrated sulfuric acid to form oleum, also called fuming sulfuric acid, and dilute sulfuric acid is then added to give concentrated sulfuric acid.
Concentrated sulfuric acid is a dense, viscous liquid that contains about 98 % sulfuric acid by mass. It is strongly hygroscopic, which means that it readily absorbs water, so it is used as a drying agent for gases.
It is also a dehydrating agent, removing hydrogen and oxygen from substances in the form of water. When concentrated sulfuric acid is added to sugar, water is removed from the sugar molecules and black carbon is left behind.
\mathrm{C_{12}H_{22}O_{11}} \longrightarrow 12\mathrm{C} + 11\mathrm{H_2O}
To dilute concentrated sulfuric acid, add the acid to water a little at a time. Adding water to the concentrated acid instead is dangerous, because so much heat is given out that the liquid can spatter.