Lewis structures model covalent bonds by using element symbols and dots around the symbol to represent valence electrons. Lines represent covalent bonds between atoms.
Gilbert Newton Lewis, an American chemist, developed a model called a Lewis structure that represents covalent bonds and nonbonding electrons with symbols, dots, and lines. A Lewis structure shows each element's symbol with dots around it to represent the element's valence electrons. Electrons that are not in an atom's valence shell, known as core electrons, are not shown in Lewis structures. These electrons, which are in orbitals with the same principal quantum number (), which indicates the relative energy of the orbital, do not have enough energy to form a bond. Hydrogen, for example, is represented in a Lewis structure with the symbol H and one dot because an uncharged hydrogen atom has one valence electron. Helium is represented with the symbol He and two dots. The first four dots are placed to the left of, to the right of, above, and below the symbol. (Helium is an exception with two dots on one side to indicate that it has a filled valence shell.) Starting from the fifth dot, the dots start pairing to form dot pairs. In Lewis structures, Lewis dot symbols are drawn together to represent molecules as defined by the octet rule, which states that atoms tend to share or donate electrons such that the valence shell contains eight electrons. An electron that is part of a covalent bond is called a bonding electron. A valence electron pair that does not form a bond is called a lone pair, or nonbonding electrons. Bonding electrons can be represented in a Lewis structure either by two dots between atoms or by a line between atoms. Hydrogen and helium are special cases in Lewis structures. Hydrogen completes its valence shell with two electrons, instead of the eight predicted by the octet rule. The same is true for helium, except helium already has a full valence shell. Helium molecules (He2) are very rare.
A double bond can be shown by two columns of shared dots or by double lines. A triple bond can be represented by three columns of dots or by triple lines. Oxygen has six valence electrons. In an oxygen molecule (O2), two oxygen atoms share two pairs of electrons to satisfy the octet rule. Nitrogen has five valence electrons. In a nitrogen molecule (N2), two nitrogen atoms share three pairs of electrons.
Lewis Structures of Small Molecules
Elements that commonly form double or triple bonds to satisfy the octet rule are oxygen, carbon, and nitrogen. Lewis structures can be used to model organic molecules with single, double, or triple carbon-carbon bonds.
Lewis Structures of Double and Triple Bonds
A free radical is an atom, molecule, or ion with one or more unpaired electrons. Single-atom versions of many substances, such as atomic oxygen or atomic chlorine, are free radicals. A free radical often forms when an existing molecule breaks down. Free radicals are very reactive. In biological systems free radicals readily bond with nearby molecules, disrupting normal function. A notable free radical is the hydroxyl radical. The hydroxyl radical is a neutral molecule consisting of one oxygen atom and one hydrogen atom. It is different from a hydroxide ion (OH–), which also has one oxygen atom and one hydrogen atom but carries a charge. The Lewis structure of the hydroxyl radical has a dot representing its unpaired electron. Free radicals containing a single unpaired electron are represented with a single dot. In the common representation of a free radical, none of the other valence electron pairs are shown. Some existing molecules have more electrons than the octet rule predicts. Examples are sulfur hexafluoride (SF6), phosphorus pentachloride (PCl5), and various noble gas compounds, such as xenon difluoride (XeF2) and xenon tetrafluoride (XeF4). In the phosphorus pentachloride (PCl5) molecule, each of the fluorine atoms has a full octet. However, the central phosphorus has 10 valence electrons. A molecule or ion that has more than an octet of valence electrons is termed hypervalent. Hypervalent atoms or centers all have somewhat large (period 2 or below) central atoms, often surrounded by more electronegative atoms, such as fluorine or chlorine.
Lewis Structures of Carbon Compounds
There is no consensus on how hypervalent molecules form. Both phosphorus and sulfur have empty 3d orbitals, and scientists have long thought that these play a role. However, recent research results disagree with this idea.