Difference Between Sulfur and Sulfate

Main Difference – Sulfur vs Sulfate

The abundance of sulfur on earth has been determined as 0.042%. Although it is a very little amount compared to elements like oxygen and silicon, it is the 13^th most abundant element on earth. Sulfate is an anion composed of a sulfur atom. But there is a distinct difference between sulfur and sulfate. The main difference between sulfur and sulfate is that sulfur is an element whereas sulfate is an anion.

Key Areas Covered

1. What is Sulfur
      – Chemical and Physical Properties, Allotropes
2. What is Sulfate
      – Chemical and Physical Properties
3. What is the Difference Between Sulfur and Sulfate
      – Comparison of Key Differences

Key Terms: Allotropes, Nonmetal, Monoclinic Sulfur, Orthorhombic Sulfur, Rhombic Sulfur, Silicon, Sulfate, Sulfur, Valency

What is Sulfur

Sulfur is an element. The atomic number of sulfur is 16 with symbol “S” and the electron configuration of sulfur is 1s²2s²2p⁶3s²3p⁴. The outermost electron containing orbital of this element is p orbital. Therefore, sulfur is grouped in p block.  According to the electron configuration of sulfur, the valency of sulfur is 6. That means, it can form maximum of 6 bonds around it. Thus, sulfur can have oxidation states up to +6. But sometimes, sulfur can gain two more electrons to fill up its p orbitals and then obtains the oxidation state -2.

Normally, sulfur is found as a solid at room temperature. But this solid does not contain pure sulfur atoms. Most of the times, the sulfur solid is made out of S₈ units. These are called sulfur crystals. One S₈ unit is composed of 8 sulfur atoms. It is a cyclic structure and is very stable. The solid sulfur appears in bright yellow color.

Figure 1: Sulfur Crystals

The melting point of sulfur is about 115⁰C. Atomic mass of sulfur is 32 g/mol. Sulfur is a nonmetal. The sulfur solid is a brittle structure since it is made of crystals. However, sulfur is insoluble in water.

Sulfur has a number of allotropes. These allotropes of sulfur differ from each other according to their composition of sulfur atoms, and their structures. Some of the common allotropes are shown below.

Allotropes of Sulfur

Orthorhombic Sulfur

The S₈ unit is in a “crown” structure. 

Rhombic Sulfur

This is a stable solid and is composed of S₈ units. The geometry of the S₈ units is rhombic.  

Monoclinic Sulfur

These are needle shaped structures.

However, it is easy to obtain one allotrope from another allotrope by heating the sample of sulfur slowly and steadily.

What is Sulfate

Sulfate is an anion, composed of a sulfur atom and four oxygen atoms. The overall charge of this anion is -2. The molecular formula for sulfate is SO₄^2-. There, two oxygen atoms are bonded to the sulfur atom through double bonds and other two are bonded to sulfur through single bonds. The oxidation state of sulfur in sulfate is +6. It is the maximum oxidation state that sulfur can have. Therefore, the sulfate cannot undergo oxidation reactions.

However, when the bond lengths in sulfate are experimentally determined, it was found that all the bond lengths are the same. Therefore, it indicates that sulfate has resonance structures.

Figure 2: Resonance Structures of Sulfate

The molar mass of sulfate anion is about 96 g/mol. Most sulfate compounds are soluble in water. But solubility of some sulfates are poor or they do not dissolve at all. For example, BaSO₄ (Barium Sulfate) is insoluble in water. Some sulfate compounds are colorless while some are very colorful when they are in the water. For example, CuSO₄ (copper sulfate) is bright blue color when it is in aqueous solution but it is white colored in the absence of water molecules.

Difference Between Sulfur and Sulfate

Definition

Sulfur: Sulfur is an element with the symbol S.

Sulfate: Sulfate is an anion, composed of a sulfur atom (S) and four oxygen atoms (O).

Mass

Sulfur: The atomic mass of sulfur is about 32 g/mol.

Sulfate: The molar mass of sulfate is about 96 g/mol.

Geometry

Sulfur: The geometry of sulfur can be orthorhombic, rhombic or monoclinic.

Sulfate: The geometry of sulfate is tetrahedral.

Composition

Sulfur: Solid Sulfur is composed of S₈ units.

Sulfate: Sulfate is composed of sulfur and oxygen atoms.

Melting Point

Sulfur: The melting point of sulfur is about 115⁰C.

Sulfate: The melting point of sulfate depends on the cation that is bound to it. 

Electrical Charge

Sulfur: Sulfur is neutrally charged.

Sulfate: Sulfate is negatively charged.

Conclusion

Sulfur atom can be found in various types of compounds or as sulfur allotropes. Sulfur allotropes are composed only of sulfur atoms whereas sulfur containing compounds are composed of many other elements. The main difference between sulfur and sulfate is that sulfur is an element whereas sulfate is an anion.

References:

1. “Allotropes of sulfur- Learn Chemistry.” Royal Society of Chemistry – Advancing excellence in the chemical sciences. N.p., n.d. Web. Available here. 10 July 2017. 
2. Libretexts. “Chemistry of Sulfur (Z=16).” Chemistry LibreTexts. Libretexts, 12 Dec. 2016. Web. Available here. 10 July 2017. 

Image Courtesy:

1. “Sulfur crystals 1” By Maxim Bilovitskiy – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Sulfate-resonance-2D” By Ben Mills – Own work (Public Domain) via Commons Wikimedia