To represent a molecule, you can use several formulas:

- electronic formula
- structural formula
- molecular formula
- geometric formula

For example, to represent methane gas, here are the various formulas we can use.

Electronic formula

It shows in the formula the electronic pairs between the bonds of atoms. It's called the formula of Lewis.

Structural formula

Formula more used than electronics. Electronic pairs are represented by a dash. Indicate the covalent bond between atoms.

Molecular formula

It is a more simplified representation of the molecule. Indicates atoms and their amount in the substance.

These formulas indicate how the molecule could be viewed in space. Show the angles and their links.

Carbon atom classification

We classify the carbon atom according to the number of carbons that are linked to it. Example:

In this case, the missing bonds are completed with hydrogen, because carbon must always make 4 bonds.

Note that carbons 1,4,5,6 and 8 are linked to only one carbon, so they are calledprimary carbons.
Carbon 3 is attached to two carbons, so it's called secondary carbon.
Carbon 7 is attached to three carbons, so it's called tertiary carbon.
Carbon 2 is attached to four carbons, so it's called quaternary carbon.