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Lewis notation and molecular structure

Although we have used diagrams to show the structure of molecules, there are other forms of notation that can be used, such as Lewis notation and Couper notation . Lewis notation uses dots and crosses to represent the valence electrons on different atoms. The chemical symbol of the element is used to represent the nucleus and the core electrons of the atom.

So, for example, a hydrogen atom would be represented like this:

A chlorine atom would look like this:

A molecule of hydrogen chloride would be shown like this:

The dot and cross in between the two atoms, represent the pair of electrons that are shared in the covalent bond.

Represent the molecule H 2 O using Lewis notation

  1. The electron configuration of hydrogen is 1s 1 and the electron configuration for oxygen is 1s 2 2s 2 2p 4 . Each hydrogen atom has one valence electron, which is unpaired, and the oxygen atom has six valence electrons with two unpaired.

  2. The water molecule is represented below.

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Represent the molecule HCN using Lewis notation

  1. The electron configuration of hydrogen is 1s 1 , the electron configuration of nitrogen is 1s 2 2s 2 2p 3 and for carbon is 1s 2 2s 2 2p 2 . This means that hydrogen has one valence electron which is unpaired, carbon has four valence electrons, all of which are unpaired, and nitrogen has five valence electrons, three of which are unpaired.

  2. The HCN molecule is represented below. Notice the three electron pairs between the nitrogen and carbon atom. Because these three covalent bonds are between the same two atoms, this is a triple bond.

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Represent the molecule H 2 S using Lewis notation

  1. Hydrogen has an electron configuration of 1s 1 and sulfur has an electron configuration of 1s 2 2s 2 2p 6 3s 2 3p 4 . Each hydrogen atom has one valence electron which is unpaired, and sulfur has six valence electrons. Although sulfur has a variable valency, we know that the sulfur will be able to form 2 bonds with the hydrogen atoms. In this case, the valency of sulfur must be two.

  2. The H 2 S molecule is represented below.

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Another way of representing molecules is using Couper notation . In this case, only the electrons that are involved in the bond between the atoms are shown. A line is used for each covalent bond. Using Couper notation, a molecule of water and a molecule of HCN would be represented as shown in figures [link] and [link] below.

A water molecule represented using Couper notation
A molecule of HCN represented using Couper notation

Dative covalent bonds

A dative covalent bond (also known as a coordinate covalent bond) is a description of covalent bonding between two atoms in which both electrons shared in the bond come from the same atom. This happens when a Lewis base (an electron donor) donates a pair of electrons to a Lewis acid (an electron acceptor). Lewis acids and bases will be discussed in [link] in chapter [link] .

One example of a molecule that contains a dative covalent bond is the ammonium ion (NH 4 + ) shown in the figure below. The hydrogen ion H + does not contain any electrons, and therefore the electrons that are in the bond that forms between this ion and the nitrogen atom, come only from the nitrogen.

Atomic bonding and lewis notation

  1. Represent each of the following atoms using Lewis notation:
    1. beryllium
    2. calcium
    3. lithium
  2. Represent each of the following molecules using Lewis notation:
    1. bromine gas (Br 2 )
    2. carbon dioxide (CO 2 )
  3. Which of the two molecules listed above contains a double bond?
  4. Two chemical reactions are described below.
    • nitrogen and hydrogen react to form NH 3
    • carbon and hydrogen bond to form a molecule of CH 4
    For each reaction, give:
    1. the valency of each of the atoms involved in the reaction
    2. the Lewis structure of the product that is formed
    3. the chemical formula of the product
    4. the name of the product
  5. A chemical compound has the following Lewis notation:
    1. How many valence electrons does element Y have?
    2. What is the valency of element Y?
    3. What is the valency of element X?
    4. How many covalent bonds are in the molecule?
    5. Suggest a name for the elements X and Y.

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Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
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