Lewis acids and bases studied chemical bonding. In 1923, Lewis wrote "An acid substance is one which can employ an electron lone pair from another molecule in completing the stable group of one of its own atoms." The Brønsted–Lowry acid–base theory was published in the same year. The two theories are distinct but complementary. A Lewis base is also a Brønsted–Lowry base, but a Lewis acid doesn't need to be a Brønsted–Lowry acid. The classification into hard and soft acids and bases (HSAB theory) followed in 1963. The strength of Lewis acid-base interactions, as measured by the standard enthalpy of formation of an adduct
Chemical composition Chemical composition Chemical composition refers to the identity and relative number of the chemical elements that make up any particular compound. The chemical composition of a pure substance corresponds to the relative amounts of the elements that constitute the substance itself. It can be expressed with a chemical formula, such as an empirical or molecular formula. For example, the chemical formula for water is HO: this means that each molecule of water is constituted by 2 atoms of hydrogen (H) and 1 atom of oxygen (O). Thus the chemical composition of water may be interpreted as a 2:1 ratio of