Atoms do not exist in isolation. They bond together to form molecules and compounds. A chemical bond is a lasting attraction between atoms that enables the formation of compounds. The bond may be ionic, covalent, or metallic. It may be strong or weak. It may be polar or nonpolar. The type of bond determines the properties of the substance.

Ionic bonds form when one atom transfers electrons to another, creating oppositely charged ions that attract each other. Sodium chloride is an example. Ionic compounds tend to be hard, brittle, and soluble in water. They conduct electricity when molten or dissolved. Covalent bonds form when atoms share electrons. Water, methane, and DNA are covalent. Covalent compounds tend to have lower melting points and do not conduct electricity. Metallic bonds form when metal atoms share a sea of delocalized electrons. They give metals their conductivity, malleability, and luster.

Bonds are not static. They vibrate, stretch, and bend. They can break and reform. The strength of a bond is measured by bond energy, the energy required to break it. Bond strength varies widely. The triple bond in nitrogen is one of the strongest, which is why nitrogen gas is so unreactive. The hydrogen bond in water is much weaker, but it is responsible for water's high boiling point and surface tension.

Chemical bonds are the glue of the material world. They determine how atoms combine, how molecules behave, and how reactions proceed.