Covalent Bonds

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A covalent bond's definition is "the sharing of a pair of electrons between adjacent atoms". In all molecules, the outermost electrons determine how its atoms interact, the electron arrangement is most stable when they occupy the innermost shells, hence the atoms fill shells in order of inner to outer (with a few exceptions). A filled electron shell is also more stable than an unfilled shell, and so in partially filled shells, covalent bonding often occurs between atoms[1]. The characteristics of a molecule are influenced by its electron arrangement. Out of all of the bonds that form between atoms, a covalent bond is the strongest to form, and the making and breaking of covalent bonds are often controlled by enzymes in living cells[2]. For instance, the bond between two chlorine atoms is also called a covalent bond. Covalent bonds divided into two groups such as polar covalent bond and nonpolar covalent bond. A chemical bond where a pair of electrons is unequally shared between two atoms is known aspolar covalent bonding, these are important in biology as they create permanent dipoles allowing electrostatic interaction between molecules, while nonpolar covalent bonding happens when a pair of electrons shared equally by two atoms[3]. There is often unequal distribution of electrons in a covalent bond, and usually the more electronegative atom pulls the electrons towards itself more strongly, hence the molecule has electrostatic potential, and the molecule is polar, allowing for properties such as hydrogen bonding[4]. Shared electrons form electron clouds, for example in hydrogen their single electrons form a covalent bond as they form a negatively charged cloud that is densest between the two positively charged nuclei, this opposes the repulsion between like charges and helps to hold them together, the balance between negative and positive charges results in characteristic bond lengths. There are also different types of covalent bond, single bonds with one shared pair of electrons, it has the smallest density of the types of covalent bond. Covalent double bonds also exist, this involves the sharing of 2 electrons each from each participating atom, this results in a shorter bond length with greater strength than a single bond, they have a characteristic geometry and often influence stereoisomerism. In some molecules, electrons are shared among three or more atoms, benzene for example, producing an intermediate bond length[5].

Reference

  1. Taken from Molecular Biology of the Cell, Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, Peter Walter. 2008, 5th ed. Published by Garland Science NY.
  2. Berg, J. Stryer, L. Tymoczko,J. Biochemistry, 7th Edition, New York: W.H Freeman and Company, Chapter 1, Page 7
  3. Polar and Nonpolar covalent bonds: Definitions and examples.(n.d.).Retrieved from http://study.com/academy/lesson/polar-and-nonpolar-covalent-bonds-definitions-and-examples.html
  4. Taken from Chemistry Libre Texts,https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Chemical_Bonding/Fundamentals_of_Chemical_Bonding/Covalent_Bonds, 11/08/2015, accessed 04/12/17, Camy Fung, Nima Mirzaee
  5. Molecular Biology of the Cell, 5th ed. Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, Peter Walter. Garland science 2008 NY
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