Sulfur (S) is a nonmetal located in Group 16 of the periodic table. It possesses six valence electrons in its outermost shell. A covalent bond forms when atoms share these valence electrons to achieve a stable configuration. The question of how many covalent bonds sulfur forms does not have a single answer, as the element displays remarkable flexibility depending on the molecule it is part of.
Sulfur’s Standard Bonding Capacity
The simplest prediction for sulfur’s bonding capacity is derived from the octet rule, which suggests atoms strive to have eight electrons in their valence shell. Since sulfur has six valence electrons, it requires two additional electrons to complete its octet. This means sulfur’s most common bonding capacity is two covalent bonds. This two-bond structure is seen in compounds like hydrogen sulfide (\(\text{H}_2\text{S}\)) and in organic compounds called thiols. In this state, the sulfur atom uses two valence electrons for bonding, leaving the remaining four electrons as two non-bonding lone pairs.
The Mechanism of Variable Bonding
Sulfur is a Period 3 element, which allows it to deviate from the two-bond standard. Unlike Period 2 elements such as oxygen, sulfur has accessible 3d-orbitals in addition to its 3s and 3p valence orbitals. This structural feature allows sulfur to accommodate more than eight valence electrons, a phenomenon known as an expanded octet. The number of covalent bonds sulfur forms can increase to four or six when electrons are promoted to these empty d-orbitals, making more unpaired electrons available for sharing. This variable bonding capacity is generally observed when sulfur bonds to highly electronegative atoms, such as oxygen or fluorine.
Covalent Bonding in Common Sulfur Compounds
The elemental form of sulfur, \(\text{S}_8\), illustrates the standard two-bond capacity. In this puckered ring structure, each sulfur atom forms two single covalent bonds, completing its octet.
Four Covalent Bonds
A compound like sulfur dioxide (\(\text{SO}_2\)) shows the capacity for four covalent bonds. The central sulfur atom is attached to two oxygen atoms, typically forming a combination of single and double bonds that results in four bonds total around the sulfur atom.
Six Covalent Bonds
The central sulfur atom in sulfuric acid (\(\text{H}_2\text{SO}_4\)) exhibits a bonding capacity of six covalent bonds. Sulfur hexafluoride (\(\text{SF}_6\)) is another clear example, where the sulfur atom forms six covalent bonds, one with each of the six fluorine atoms. These examples demonstrate that sulfur’s covalent bond count can be 2, 4, or 6.