The simplest carbohydrates are called monosaccharides, which have a carbon chain length of between three and seven. Many simple sugars are monosaccharides, including glucose and fructose. Note that glucose and fructose have the same chemical formula, but are arranged differently.
There are three main groups of carbohydrates. Monosaccharides are the shortest and simplest, and are the building blocks of more complex carbohydrates. Disaaccharides consist of precisely two monosaccharides bonded together covalently, typically at the end. Finally, polysaccharides are the most complex carbohydrates, consisting of three or more monosaccharides bonded together. Common examples of polysaccharides are starch and glycogen.