D7.3 Lewis Structures for Covalent Molecules

Molecular orbitals for molecules with three or more atoms are complicated and hard to draw. Thus, although MOs would convey a more descriptive and accurate picture of electron distribution within a molecule, chemists often rely on simpler diagrams to depict the covalent bonding. It will aid your understanding of chemistry if you can connect these simpler diagrams mentally with the more complete picture given by MOs.

The most commonly used hand-drawn depiction is the Lewis structure: a diagram that represents atomic nuclei and core electrons by chemical symbols and valence electrons as dots or lines. A Lewis structure is built by combining Lewis diagrams of the constituent atoms.

Activity: Lewis Diagrams

In Lewis structures, a single covalent bond is drawn as a pair of electron dots shared between two adjacent atoms, a bond pair. Valence electrons that are not in a bond are shown as pairs of dots associated with individual atoms, lone pairs. For example:

A Lewis dot diagram shows a reaction. Two chlorine symbols, each surrounded by seven dots are separated by a plus sign. The dots on the first atom are all blue and the dots on the second atom are all read. A right-facing arrow points to two chlorine symbols, each with six dots surrounding their outer edges and a shared pair of dots in between.

In the Cl2 molecule, each Cl atom has three lone pairs and the two Cl atoms share one bond pair. Hence, each Cl atom in Cl2 has formed an octet (is surrounded by eight valence electrons).

For simplicity and clarity, a bond pair is typically represented by a line instead of a pair of dots:

Two sets of Lewis structures are shown. The left-hand structures show two H atoms connected by a single bond and two Cl atoms connected by a single bond and each surrounded by three lone pairs. The right-hand structures show two H atoms connected by a bond pair and two Cl atoms connected by a bond pair and each surrounded by three lone pairs.

Activity: Lewis Structure and Electron Sharing

The Octet Rule

The octet rule states that atoms of main-group elements tend to gain, lose, or share enough electrons to form an octet (eight valence electrons). Such noble-gas electron configurations with completely filled valence shells are more stable, and therefore should correspond to how the electrons are arranged in a molecule.

The Lewis diagram for an atom can be used to predict the number of bonds the atom will form. For example, a carbon atom has four valence electrons and therefore requires four more electrons to reach an octet:

It is important to keep in mind that it is impossible to exceed an octet for atoms in the second period. This is particularly relevant because you will encounter numerous molecules containing the elements C, N and O.

Finally, because a hydrogen atom needs only two electrons to fill its valence shell, H is an important exception to the octet rule and forms only one bond.

Exercise: Number of Bonds

Double and Triple Bonds

Two atoms may need to share more than one pair of electrons to achieve the requisite octet. In other words, the bond order is greater than 1. A double bond consists of two pairs of electrons being shared between two atoms. For example:

Two pairs of Lewis structures are shown. The left pair of structures shows a carbon atom forming single bonds to two hydrogen atoms. There are four electrons between the C atom and an O atom. The O atom also has two pairs of dots. The word “or” separates this structure from the same diagram, except this time there are two bond lines between the C atom and O atom. The name, “Formaldehyde” is written below these structures. Two more structures are on the right. The left shows two C atoms with four dots in between them and each C atom forming single bonds to two H atoms. The word “or” precedes the second structure, which is the same except that the C atoms are connected by two bond lines. The name, “ethene” is written below these structures.

A triple bond forms when three pairs of electron are shared between two atoms. For example:

Two pairs of Lewis structures are shown. The left pair of structures show a C atom and an O atom with six dots in between them and a lone pair on each. The word “or” and the same structure with a triple bond in between the C atom and O atom also are shown. The name “Carbon monoxide” is written below these structures. The right pair of structures show a C atom and an N atom with six dots in between them, a lone pair on N, and a bond pair between the C atom and a H atom. The word “or” and the same structure with a triple bond in between the C atom and N atom also are shown. The name “hydrogen cyanide” is written below these structures.

Activity: Double and Triple Bonds

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Chemistry 109 Fall 2021 by John Moore, Jia Zhou, and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.