A chemical bond forms when atoms interact to achieve a more stable electron configuration, typically by filling their outermost electron shell. When two atoms of selenium (Se) bond together, the resulting interaction is a nonpolar covalent bond. This specific type of bond involves the mutual sharing of electrons between the two identical atoms. Since both atoms have an equal pull on the shared electrons, the electrical charge distribution remains perfectly balanced.
Selenium’s Place in the Periodic Table
Selenium is a nonmetal element found in Group 16 of the periodic table, a column often referred to as the Chalcogens. Elements in this group, which also includes oxygen and sulfur, share similar chemical tendencies. Selenium atoms possess six valence electrons in their outermost shell.
According to the octet rule, atoms strive to complete their valence shell with a total of eight electrons for maximum stability. With only six electrons, a selenium atom requires an additional two electrons to achieve this stable configuration.
Since selenium is a nonmetal, it needs two electrons to satisfy the octet rule. When two identical nonmetal atoms bond, neither can fully strip electrons from the other, preventing the formation of an ionic bond. Instead, the shared need for two additional electrons leads to electron sharing.
The Mechanism of Covalent Bonding
Covalent bonds form between nonmetal atoms, and because the two selenium atoms are identical, the electronegativity difference is zero. This zero difference defines a nonpolar covalent bond, ensuring the shared electrons are distributed evenly between the two nuclei.
To satisfy the need for two electrons, the two selenium atoms mutually share two pairs of valence electrons (four shared electrons). This sharing results in a double bond, represented as \(\text{Se}=\text{Se}\). The double bond ensures each selenium atom effectively counts eight electrons: the four shared electrons and the four unshared electrons (two lone pairs) remaining on each atom.
The resulting \(\text{Se}_2\) molecule, which exists primarily in the gas phase, is a simple model of this bonding mechanism. The arrangement of shared and unshared electrons minimizes repulsive forces while fulfilling the octet rule for both atoms. The bond is maintained by the strong electrostatic attraction between the positively charged nuclei and the shared electron pairs.
Structural Forms of Elemental Selenium
While the simplest unit is the diatomic \(\text{Se}_2\) molecule, elemental selenium commonly exists in more complex solid structural forms called allotropes. These allotropes, including red, black, and metallic gray selenium, are all held together by the same fundamental covalent bonds.
The red and black allotropes are composed of puckered, single-bonded rings, such as the \(\text{Se}_8\) structure. In these rings, each selenium atom is covalently bonded to two neighbors, maintaining a stable configuration. The average selenium-selenium bond distance within the \(\text{Se}_8\) ring is approximately 233.5 picometers.
The most stable form, gray selenium, is characterized by infinite, helical polymeric chains. In these chains, each atom forms a single covalent bond with two adjacent atoms. The selenium-selenium bond length is about 237.3 picometers, and the bond angle is approximately 103.1 degrees.