# Lewis Structures

## Lewis structures

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 inner electrons of the atom. To determine which are the valence electrons we look at the last energy level in the atom’s electronic structure (Chemistry 101). For example, chlorine’s electronic structure can be written as: $$1\text{s}^{2}2\text{s}^{2}2\text{p}^{6}3\text{s}^{2}3\text{p}^{5}$$ or $$[\text{Ne}]3\text{s}^{2}3\text{p}^{5}$$. The last energy level is the third one and it contains 7 electrons. These are the valence electrons.

#### Tip:

If we write the condensed electron configuration, then we can easily see the valence electrons.

For example:

A hydrogen atom (one valence electron) would be represented like this: A chlorine atom (seven valence electrons) 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.

The table below gives some further examples of Lewis diagrams.

 Iodine $$\text{I}_{2}$$  Water $$\text{H}_{2}\text{O}$$  Carbon dioxide $$\text{CO}_{2}$$  Hydrogen cyanide $$\text{HCN}$$  Table: Lewis diagrams for some simple molecules

For carbon dioxide, you can see how we represent a double bond in Lewis notation. As there are two bonds between each oxygen atom and the carbon atom, two pairs of valence electrons link them. Similarly, hydrogen cyanide shows how to represent a triple bond.