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

Consider each element listed below. In your course notebook write the electron configuration for an atom of each element, determine the number of valence electrons, and draw a Lewis diagram. How are the Lewis diagrams related to the position of each element in the periodic table?

N      C      S      As      O      Br      F      Si      H

After writing and drawing in your notebook, left-click here for a comparison.

Lewis diagrams for atoms in the same vertical column (group) in the periodic table have the same number of dots. The number of dots (number of valence electrons) equals the group number in the periodic table or the group number minus 10. For example As is in group 15 (VA) so the number of valence electrons is 5.

A series of Lewis diagrams with a number below each. Col 1: N with dot above, dot to right, two dots below, dot to left; number 5 below. C with dot above, dot to right, dot below, dot to left; 4 below. Col 3: S with two dots above, dot to right, two dots below, dot to left; 6 below. Col 4: As with dot above, dot to right, dot below, two dots to left; 5 below. Col 5: O with two dots above, dot to right, two dots below, dot to left; 6 below. Col 6: Br with two dots above, two dots to right, dot below, two dots to left; 7 below. Col 7: F with two dots above, two dots to right, two dots below, one dot to left; 7 below. Col 8: Si with dot above, dot to right, dot below, dot to left; 4 below. Col 9: H with one dot to right. 1 below.

The Lewis diagram for each atom is shown above with the number of valence electrons below the atom symbol.

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

Write answers to these questions in your course notebook:

  1. Write a Lewis structure for N2 and a Lewis structure for O2. Describe the type of bond in each case.
  2. Do the N atoms in N2 and the O atoms in O2 follow the rule for number of bonds from the above Exercise: Number of Bonds?
  3. Use the molecular-orbital energy level diagram to calculate the bond order for N2 and for O2. How do the bond orders relate to the Lewis structures?
Write in your notebook, then left-click here for an explanation.

Each N atom forms three bonds (the three electron pairs in the triple bond) and follows the rule for number of bonds. The Lewis structure for N2 also has one lone pair on each N atom.

Each O atom forms two bonds (the two electron pairs in the double bond) and follows the rule for number of bonds. The Lewis structure for O2 also has two lone pairs on each O atom.

The MO diagram for N2 yields a bond order of 3. This corresponds with a triple bond in the Lewis structure.

The figure shows MO energy level diagram for N\(_2\) above its Lewis structure. In the MO diagram for N\(_2\) there are pairs of electron arrows, one pointing up, one pointing down, in all MOs up to pi 2 p. The Lewis structure for N\(_2\) has three bond lines between the N symbols and one lone pair on each N. From the MO diagram the bond order of N\(_2\) is three, which corresponds to the number of bond lines in the Lewis diagrams.

The MO diagram for O2 gives a bond order of 2. This corresponds with a double bond in the Lewis structure. (Note that the Lewis structure does not predict two unpaired electrons, but the MO diagram does.)

The figure shows MO energy level diagram for O\(_2\) above its Lewis structure. In the MO diagram for O\(_2\) there are pairs of electron arrows, one pointing up, one pointing down, in all MOs up to pi-star 2 p. The two pi-star 2 p MOs each have one electron arrow pointing up. The Lewis structure for O\(_2\) has two bond lines between the O symbols and two lone pairs on each O. From the MO diagram the bond order of O\(_2\) is two, which corresponds to the number of bond lines in the Lewis diagrams.

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Chem 109 Fall 2023 Copyright © by Jia Zhou; John Moore; and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.