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Chemical Bonding: Lewis structures
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1 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 dierent 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
A chlorine atom would look like this:
Figure 2
A molecule of hydrogen chloride would be shown like this:
Figure 3
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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 (Solution on p. 4.) Represent the molecule H 2 O using Lewis notation Exercise 2: Lewis notation: Molecules with multiple bonds (Solution on p. 4.) Represent the molecule HCN (hydrogen cyanide) using Lewis notation Exercise 3: Lewis notation: Atoms with variable valencies (Solution on p. 4.) Represent the molecule H 2 S (hydrogen sulphide) using Lewis notation
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 gures Figure 4 and Figure 5 below.
Figure 4: A water molecule represented using Couper notation
Figure 5: A molecule of HCN represented using Couper notation
1.1 Atomic bonding and Lewis notation
- Represent each of the following atoms using Lewis notation: a. beryllium b. calcium c. lithium Click here for the solution.^1
- Represent each of the following molecules using Lewis notation: a. bromine gas (Br 2 ) b. carbon dioxide (CO 2 ) Click here for the solution.^2 (^1) http://www.fhsst.org/lOC (^2) http://www.fhsst.org/lOa
Solutions to Exercises in this Module
Solution to Exercise (p. 2)
Step 1. The electron con guration of hydrogen is 1 s^1 and the electron con guration for oxygen is 1 s^22 s^22 p^4. Each hydrogen atom has one valence electron, which is unpaired, and the oxygen atom has six valence electrons with two unpaired.
Figure 7
Step 2. The water molecule is represented below.
Figure 8
Solution to Exercise (p. 2)
Step 1. The electron con guration of hydrogen is 1 s^1 , the electron con guration of nitrogen is 1 s^22 s^22 p^3 and for carbon is 1 s^22 s^22 p^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 ve valence electrons, three of which are unpaired.
Figure 9
Step 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.
Figure 10
Solution to Exercise (p. 2)
Step 1. Hydrogen has an electron con guration of 1 s^1 and sulphur has an electron con guration of 1 s^22 s^22 p^63 s^23 p^4. 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 11
Step 2. The H 2 S molecule is represented below.
Figure 12
