What are carbohydrates composed of?

The chemical composition of carbohydrates - as analysed in the nineteenth century - is a combination of carbon (C), hydrogen (H) and water (H2O). Many carbs have the general chemical formula Cx(H2O)x, but the class is too large to fit into a simple chemical formula. Carbohydrates are often isomers - meaning, they have the same atomic composition but different structures. Fructose, galactose and glucose are isomers with the chemical formula C6H12O6. There are many classification schemes for carbohydrates. The most common one separates them into 4 major groups: monosaccharides, disaccharides, oligosaccharides and polysaccharides.