The Chemistry of Sulfur

Atomic Number: 16
Mass Number: 32.06
Electron Configuration: 1s22s22p63s23p4
Melting Point: 119oC
First Ionization Energy: 1004 kJ/mol
Electronegativity: 2.6
Atomic Radius: 104 pm
Ionic Radius: 184 pm (S2-)

Allotropes

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 190oC, the liquid becomes mobile again as the chains break apart. If this liquid is suddely cooled, and amorphous substance results known as plastic sulfur. The blue color of sulfur vapor is due to diatomic sulfur. Like diatomic oxygen, diatomic sulfur it is paramagnetic.

Occurrence

Many different metals, especially transition metals, occur as sulfide ores. Some examples include pyrite (FeS2), galena (PbS), sphalerite (ZnS), cinnabar (HgS), and chalcocite (Cu2S). Iron pyrite is known as fool's gold because of its characteristic lustor. In this compound the sulfur is in the form of the disulfide ion (S22-), which is analagous to the peroxide ion. 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.

Oxygen Compounds

The two most common molecular compounds between sulfur and oxygen are sulfur dioxide and sulfur trioxide. Sulfur dioxide has a pungent, choking odor and is the product formed from the combustion of sulfur. 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. The combustion of coal which contains large amounts of sulfur is a very serious problem due to this reaction. The sulfur dioxide dissolves in atmospheric moisture and comes down as "acid rain."

SO2 + H2O = H2SO3

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.

SO3 + H2O = H2SO4

 Formula Name Melting Point Boiling Point
SO2 sulfur dioxide -75.5oC -10oC
SO3 sulfur trioxide 16.9oC 44.6oC

Sulfides and Polysulfides

Sulfur form an extensive series of anions, summarized in the table below. As might be expected, sulfur has a tendency to gain two electrons and form the sulfide ion. Sulfur also forms two diatomic ions, analagous to the peroxide and superoxide ions. Last, sulfur forms two polysulfide ions.The color of the stone lapis lazuli, which was used by the ancient Egyptians, is due to the presence of polysulfide ions.

S2- sulfide ion
S22- disulfide ion; analogous to the peroxide ion
S2- anlogous to the superoxide ion; paramagnetic; green in color
S3- paramagnetic, contains an odd number of electrons; blue in color
S32- a polysulfide; bent shape

Reactions of Sulfur

Elemental sulfur undergoes a number of reactions. Sulfur burns with a blue flame to produce sulfur dioxide (Figure 5). It combines with fluorine to form sulfur hexafluoride, with chlorine to produce sulfur dichloride and disulfur dichloride. It aso with metals such as zinc and magnesium to form sulfides (Figure 6) and with hydrogen to form hydrogen sulfide.
Figure 1. Elemental sulfur
Figure 2. Pyrite, FeS2
Figure 3. Galena, PbS
Figure 4. Cinnabar, HgS
Photo courtesy of R. Weller, Cochise College

Figure 5. Burning sulfur
Figure 6. Zinc and sulfurr reaction
View a video of this reaction here.