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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:

Figure 1
Figure 1 (CG11C1_006.png)

A chlorine atom would look like this:

Figure 2
Figure 2 (CG11C1_007.png)

A molecule of hydrogen chloride would be shown like this:

Figure 3
Figure 3 (CG11C1_008.png)

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

Exercise 1: Lewis notation: Simple molecules

Represent the molecule H2OH2O using Lewis notation

Solution

  1. Step 1. For each atom, determine the number of valence electrons in the atom, and represent these using dots and crosses. :

    The electron configuration of hydrogen is 1s11s1 and the electron configuration for oxygen is 1s22s22p41s22s22p4. Each hydrogen atom has one valence electron, which is unpaired, and the oxygen atom has six valence electrons with two unpaired.

    Figure 4
    Figure 4 (CG11C1_009.png)
  2. Step 2. Arrange the electrons so that the outermost energy level of each atom is full. :

    The water molecule is represented below.

    Figure 5
    Figure 5 (CG11C1_010.png)

Exercise 2: Lewis notation: Molecules with multiple bonds

Represent the molecule HCNHCN (hydrogen cyanide) using Lewis notation

Solution

  1. Step 1. For each atom, determine the number of valence electrons that the atom has from its electron configuration. :

    The electron configuration of hydrogen is 1s11s1, the electron configuration of nitrogen is 1s22s22p31s22s22p3 and for carbon is 1s22s22p21s22s22p2. 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.

    Figure 6
    Figure 6 (CG11C1_011.png)
  2. Step 2. Arrange the electrons in the HCNHCN molecule so that the outermost energy level in each atom is full. :

    The HCNHCN 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.

    Figure 7
    Figure 7 (CG11C1_012.png)

Exercise 3: Lewis notation: Atoms with variable valencies

Represent the molecule H2SH2S (hydrogen sulphide) using Lewis notation

Solution

  1. Step 1. Determine the number of valence electrons for each atom. :

    Hydrogen has an electron configuration of 1s11s1 and sulphur has an electron configuration of 1s22s22p63s23p41s22s22p63s23p4. Each hydrogen atom has one valence electron which is unpaired, and sulphur has six valence electrons. Although sulphur has a variable valency, we know that the sulphur will be able to form 2 bonds with the hydrogen atoms. In this case, the valency of sulphur must be two.

    Figure 8
    Figure 8 (CG11C1_013.png)
  2. Step 2. Arrange the atoms in the molecule so that the outermost energy level in each atom is full. :

    The H2SH2S molecule is represented below.

    Figure 9
    Figure 9 (CG11C1_014.png)

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 HCNHCN would be represented as shown in figures Figure 10 and Figure 11 below.

Figure 10: A water molecule represented using Couper notation
Figure 10 (CG11C1_015.png)
Figure 11: A molecule of HCNHCN represented using Couper notation
Figure 11 (CG11C1_016.png)

Atomic bonding and Lewis notation

  1. Represent each of the following atoms using Lewis notation:
    1. beryllium
    2. calcium
    3. lithium
    Click here for the solution.
  2. Represent each of the following molecules using Lewis notation:
    1. bromine gas (Br2Br2)
    2. carbon dioxide (CO2CO2)
    Click here for the solution.
  3. Which of the two molecules listed above contains a double bond?
    Click here for the solution.
  4. Two chemical reactions are described below.
    • nitrogen and hydrogen react to form NH3NH3
    • carbon and hydrogen bond to form a molecule of CH4CH4
    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
    Click here for the solution.
  5. A chemical compound has the following Lewis notation:
    Figure 12
    Figure 12 (CG11C1_018.png)
    1. How many valence electrons does element YY have?
    2. What is the valency of element YY?
    3. What is the valency of element XX?
    4. How many covalent bonds are in the molecule?
    5. Suggest a name for the elements XX and YY.
    Click here for the solution.

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