A group of chemical species that are rapidly interconverted by chemical reactions. For example, hydrogen atoms (H), hydroxyl radical (OH), and hydroperoxyl radical (HO2) are referred to as the “odd hydrogen” family. Other families are the NOx family (NO and NO2), and ClOx family (predominantly Cl and ClO). The concept of chemical families is often encountered in the modeling of atmospheric chemistry, where the treatment of groups of interconverting species as families can reduce the complexity of the model calculation.