SULFUR

Atomic Number: 16
Mass Number: 32.06
Electron Configuration: 1s²2s²2p⁶3s²3p⁴
Melting Point: 119^oC
First Ionization Energy: 1004 kJ/mol
Electronegativity: 2.6
Atomic Radius: 104 pm
Ionic Radius: 184 pm (S^2-)

Sulfur has four naturally occurring isotopes, as summarized below.

Elemental sulfur occurs in the form of eight-membered rings rather than as diatomic molecules. The two most important allotropes of sulfur are rhombic sulfur and monoclinic sulfur; these two forms differ in the way in wich the rings are stacked. The rhombic form is the more stable of the two. Monoclinic sulfur appears as long needles and is formed when sulfur when sulfur solidifies at the melting point. In monoclinic sulfur the eight-membered rings are not as efficiently packed and the density is slightly lower than for monoclinic sulfur.

When sulfur is melted, several changes in apperance and viscosity occur. Sulfur melts to form a mobile liquid. Continued heating results in the formation of a viscous red liquid. The viscosity is attributed to a breakdown of the eight-membered rings, followed by linking into long chains. Above temperature of 190 C, the liquid becomes mobile again as the chains break apart. If this liquid is suddely cooled, and amorphous substance known as plastic sulfur

Sulfur vapor contains both eight-membered rings and diatomic sulfur; the blue color of the vapor is due to diatomic sulfur. Like diatomic oxygen, diatmic sulfur is paramagnetic and has two unpaired electrons.

Many different metals, especially transition metals, occur as sulfide ores. Some examples include pyrite, galena, sphalerite, cinnibar, and chalcocite. Iron pyrite is known as fool's gold because of its characteristic lustor. Cinnibar is bright red in color and was once used as a pigment.

Sulfur also occurs in elemental form. Most sulfur is obtained from the Frasch process, which is used to mine elemental sulfur trapped underground. In this process, steam is passed through the outermost of three concentric pipes and used to melt the sulfur, which has a relatively low melting point. Compressed air is then forced through innermost the pipe, forcing the molten sulfur to the surface through the center pipe.

Iron Pyrite. Iron pyrite is the mineral FeS2 and is referred to as fool's gold due to the characteristic lustor and color. Although it is tempting to say the iron is in the +4 oxidation state, this is not correct. The iron actually in the +2 oxidation state, and sulfur is present as the disulfide ion. The disulfide ion is analagous to the peroxide ion and is discussed below.

Galena. Galena is the mineral lead sulfide. Like iron pyrite, it also has a lustorous appearance.

Compounds with Oxygen

The two most common molecular compounds between sulfur and oxygen are sulfur dioxode and sulfur trioxide. Sulfur dioxide has a pungent, choking odor and is the product formed when sulfur burns. Like ozone, the sulfur dioxide molecule has an angular shape, and the the bond angle is 119 degrees. Liquid sulfur dioxide is a good solvent. Sulfur dioxide dissolves in water to produce sulfurous acid, a weak diprotic acid.

Sulfur trioxide is formed from the oxidation of sulfur dioxide. In this particular molecule, the sulfur is the central atom and supports an expanded octet. Sulfur trioxide reacts vigorously with water to produce sulfuric acid.

Compounds With Fluorine and Chlorine

Unlike oxygen, sulfur forms an extensive series of compounds with fluorine. These are summarized in the table below. The compound disulfur difluoride exists as two isomers; both are unstable and very reactive. The compound sulfur tetrafluoride, is a selective fluorinating agent and reacts violently with water to produce hydrogen fluoride and sulfur dioxode. The compound sulfur hexafluoride is chemically unreactive due primarily to steric hindrance and is used as an electrical insulator.

The list of sulfur-chlorine compounds is relatively short and restricted to sulfur dichloride and disulfur dichloride. Disulfur dichloride is a a fuming orange liquid and is made by passing chlorine gas through molten sulfur. Sulfur dichloride is a fuming orange liquid and is made by further chlorinaton of disulfur dichloride.

S2F2	disulfur difluoride	colorless gas shape similar to hydrogen peroxide
S2F2	disulfur difluoride	colorless gas
SF4	sulfur tetrafluoride	boiling point -30oC colorless gas reacts violently with water geometry is seesaw-shaped
SF6	sulfur hexafluoride	colorless gas chemically unreactive octahedral geometry
S2F10	disulfur decafluoride	colorless liquid

Sulfur forms an extensive series of oxoanions these are listed in the table below.

Formula	Name	Structure	Formula	Name	Structure
SO42-	Sulfate		SO32-	Sulfite
S2O32-	Thiosulfate		S2O42-	Dithionite
S2O62-	Dithionate		S3O62-	Trithionate
S3O82-	Tetrathionate		S2O82-	Peroxydisulfate

Some of the anions formed by sulfur are listed below. As might be expected, sulfur has a tendency to gain two electrons and form the sulfide ion. Sulfur also forms the disulfide ion, which is analogous to to the peroxide ion. The only simple metal sulfides are those of the sulfide and disulfide ions. Sulfur forms two colored paramagnetic ions that are responsible for the color of the stone lapus lazuli, which was used by the ancient Egyptians.

A large nummber of metal sulfides are known, and several differnt structures are adopted. Sodium sulfide exhibits the antifluorite structure, in which the sulfide ions form a face-centered cubic array and the sodium ions reside in the tetrahedral holes. Zinc sulfide exists as both the sphalerite structure and the wurtzite structure. The sphalerite structure is cubic, with the sulfide ions forming a face-centered cubic array and the zinc ions residing in half of the tetrahedral holes. The sphalerite structure is also adopted by cadmium sulfide and mercury sulfide. The wurtzite structure is hexagonal, with the sulfide ions forming a hexagonal close-packed array and the zinc ions residing in half of the tetrahedral holes. Sulfur can also form the disulfide ion, which is analagous to the peroxide ion. Iron pyrite is best viewed as containing discrete disulfide ions. 

Elemental sulfur undergoes a number of reactions. Sulfur reacts with oxygen to produce sulfur dioxide, with fluorine to form sulfur hexafluoride, with chlorine to produce sulfur dichloride and disulfur dichloride, with metals such as zinc and magnesium to form sulfides, and with hydrogen to form hydrogen sulfide.

Burning Sulfur. Sulfur burns in air with a blue flame and produces sulfur dioxode. In this photo, a sample of burning sulfur has been lowered into a bottle of pure oxygen gas

Zinc and Sulfur. A mixture of powdered zinc and sulfur will vigorously react when ignited to produce the ionic compound zinc sulfide. Mixtures of zinc and sulfur have been used as rocket fuel. View a video of this reaction here.