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Strong and weak attractive forces between atoms are the “glue” that holds individual molecules together and permits interactions between different molecules. When two atoms share a single pair of electrons, the result is a covalent bond—a type of strong force that holds atoms together in molecules. Sharing of multiple pairs of electrons results in multiple covalent bonds (e.g., “double” or “triple” bonds). The weak attractive forces of noncovalent interactions are equally important in determining the properties and functions of biomolecules such as proteins, nucleic acids, carbohydrates, and lipids. In this section, we first review covalent bonds and then discuss the four major types of noncovalent interactions: ionic bonds, hydrogen bonds, van der Waals interactions, and the hydrophobic effect.