Covalent bond
A covalent bond is a chemical bond in which two neighbouring atoms share valence (outer) electrons.[1] Chemical bonds are interactions that hold two or more atoms together; they form because of a balance between attractive and repulsive forces, which results in an overall decrease in energy.[2]
Sharing electrons
Chemists split the idea of valency into two kinds: covalency, where electrons are shared between bonded atoms, and electrovalency, where electrons are transferred to form ions that are held together by electrostatic attraction.[3]
The chemist Gilbert N. Lewis described the covalent bond as a pair of electrons shared between two atoms.[3] A single bond shares two electrons, a double bond four and a triple bond six; they are drawn as one, two or three lines. Pairs of outer electrons that are not shared are called lone pairs.[4]
The octet rule
The octet rule states that most atoms in a Lewis structure are surrounded by no more than eight electrons, counting both shared and unshared electrons.[4] For example, in carbon tetrafluoride (CF4), carbon, which has four outer electrons, and fluorine, which has seven, each end up with eight. Hydrogen is stable with just two.[3]
There are exceptions. Hydrogen, beryllium and boron often have fewer than eight electrons; in boron trifluoride (BF3) boron has only six, and the compound is very reactive. Some molecules, called free radicals, have an unpaired electron, and elements in period 3 and below can have more than eight electrons around them.[5]
Lewis structures
A Lewis structure (also called a Lewis dot structure or Lewis diagram) shows how the valence electrons and nuclei are arranged in a covalently bonded molecule or ion.[6] To draw one, chemists count the valence electrons, arrange the atoms (hydrogen is never the central atom), join them with single bonds, add the remaining electrons, and then check the octets, forming double or triple bonds if needed.[7]
History
The idea of eight outer electrons came partly from the stability of the noble gases, which were found to have completed electron shells.[8][9] Around 1902 Lewis sketched "cubic atoms" whose cubes shared outer electrons along an edge, and in 1904 Richard Abegg described a "rule of eight".[9] The octet rule developed by Lewis and Irving Langmuir says that elements combine so as to reach the same electron arrangement as a noble gas.[8] Linus Pauling's book The Nature of the Chemical Bond was dedicated to Lewis.[10]
See also
References
- ↑ Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. p. 132. ISBN 978-1-337-61229-6.
{{cite book}}:|access-date=requires|url=(help) - ↑ Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. p. 104. ISBN 978-1-337-61229-6.
{{cite book}}:|access-date=requires|url=(help) - ↑ 3.0 3.1 3.2 Mackay, K. M.; Mackay, R. A.; Henderson, W. (2002). Introduction to Modern Inorganic Chemistry (6th ed.). Cheltenham: Nelson Thornes. section 3.1: Introduction. ISBN 0-7487-6420-8. Retrieved 2026-10-23.
- ↑ 4.0 4.1 Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. p. 134. ISBN 978-1-337-61229-6.
{{cite book}}:|access-date=requires|url=(help) - ↑ Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. p. 137. ISBN 978-1-337-61229-6.
{{cite book}}:|access-date=requires|url=(help) - ↑ Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. p. 133. ISBN 978-1-337-61229-6.
{{cite book}}:|access-date=requires|url=(help) - ↑ Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. p. 135. ISBN 978-1-337-61229-6.
{{cite book}}:|access-date=requires|url=(help) - ↑ 8.0 8.1 Brock, William H. (1992). The Fontana History of Chemistry (ePub ed.). London: Fontana Press. Chapter 9: Principles of Chemistry, section The Inert Gases. ISBN 9780007394968.
{{cite book}}:|access-date=requires|url=(help) - ↑ 9.0 9.1 Brock, William H. (1992). The Fontana History of Chemistry (ePub ed.). London: Fontana Press. Chapter 13: The Nature of the Chemical Bond, section The Lewis Atom. ISBN 9780007394968.
{{cite book}}:|access-date=requires|url=(help) - ↑ Brock, William H. (1992). The Fontana History of Chemistry (ePub ed.). London: Fontana Press. Chapter 13: The Nature of the Chemical Bond, section The Lewis Atom / Spreading the Electronic Theory. ISBN 9780007394968.
{{cite book}}:|access-date=requires|url=(help)