Isomers are compounds that have the same molecular formula. To determine whether two molecules are isomers, just count how many atoms of each type are in both molecules. If both molecules have the same count for all of the different atoms, the molecules will be isomers.
For example, consider the following molecules.
Count how many atoms of each type are in the structure on the left. Then count how many atoms of each type are in the structure on the right. Remember that each carbon needs to have a total of four
bonds, so fill in hydrogens where necessary.
The structure on the left has 7 carbons, 13 hydrogens, one bromine, and one oxygen
, or a molecular formula of C7H13BrO
. The molecule on the right has the same number of each atom and the same molecular formula. Therefore, these two molecules are isomers