Molecules can be classified in many ways. Depending on the differences in electronegativity between the bonded atoms, a molecule can be either polar or nonpolar. All molecules that contain carbon are considered organic. Those that do not are considered inorganic. There are also classes based on biochemical function and structure.
The term molecule usually refers to two or more atoms bonded together covalently. The atoms bonded can be of the same element, such as diatomic molecules. Atoms with a significant difference in electronegativity values produce polar bonds and can contribute to an overall polar molecule. Some classifications of molecules are not mutually exclusive, meaning it is possible to have a polar organic molecule or a nonpolar molecule.
In biochemistry, molecules are classified by their structure, as this often determines their function. The four major classes are carbohydrate, proteins, lipids and nucleic acids. Within each class, there are further subdivisions and types of molecules. Carbohydrates include various strings of carbon molecules called sugars. Proteins are compounds formed from any of the 20 amino acids and peptide bonds. Lipids include molecules that are oily, and do not dissolve in water or evaporate easily, such as fats and cholesterol. Nucleic acids are composed of nucleotides, and include RNA and DNA.