With few exceptions (e.g., deoxyribose), monosaccharide have this chemical formula: Cx(H2O)y, where conventionally x ≥ 3. Monosaccharides can be classified by the number x of carbon atoms they contain: triose (3) tetrose (4), pentose (5), hexose (6), heptose (7), and so on.
The most important monosaccharide, glucose, is a hexose. Examples of heptoses include the ketoses mannoheptulose and sedoheptulose. Monosaccharides with eight or more carbons are rarely observed as they are quite unstable. In aqueous solutions monosaccharides exist as rings if they have more than four carbons.
Simple monosaccharides have a linear and unbranched carbon skeleton with one carbonyl (C=O) functional group, and one hydroxyl (OH) group on each of the remaining carbon atoms. Therefore, the molecular structure of a simple monosaccharide can be written as H(CHOH)n(C=O)(CHOH)mH, where n + 1 + m = x; so that its elemental formula is CxH2xOx.
By convention, the carbon atoms are numbered from 1 to x along the backbone, starting from the end that is closest to the C=O group. Monnosaccharides are the simplest units of carbohydrates and the simplest form of sugar.
If the carbonyl is at position 1 (that is, n or m is zero), the molecule begins with a formyl group H(C=O)− and is technically an aldehyde. In that case, the compound is termed an aldose. Otherwise, the molecule has a keto group, a carbonyl −(C=O)− between two carbons; then it is formally a ketone, and is termed a ketose. Ketoses of biological interest usually have the carbonyl at position 2.