5.1 Lewis structures

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.

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 $\mathrm{HCN}$ would be represented as shown in figures [link] and [link] below.

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 ( $\mathrm{Br}{}_2$ )
   2. carbon dioxide ( $\mathrm{CO}{}_2$ )
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 $\mathrm{NH}{}_3$
   • carbon and hydrogen bond to form a molecule of $\mathrm{CH}{}_4$
   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 $\mathrm{Y}$ have?
   2. What is the valency of element $\mathrm{Y}$ ?
   3. What is the valency of element $\mathrm{X}$ ?
   4. How many covalent bonds are in the molecule?
   5. Suggest a name for the elements $\mathrm{X}$ and $\mathrm{Y}$ .