The systematic numbering system originally created by the Du Pont Chemical Company in the early 1930s, which is now the standard method in the naming of chlorofluorocarbons (CFCs). Chlorofluorocarbons, such as dichlorodifluoromethane (CCl2F2), and trichlorofluorocarbon (CCl3F), were given the nicknames CFC-12 and CFC-11, respectively, by Du Pont so that they could be remembered more easily. In order to derive the chemical structure of a chlorofluorocarbon from its common name, the rule of 90 states that you would add 90 to the number at the end of the name. In the case of CFC-12, add 90+12 to get 102. The hundreds place, the one in this example, represents the number of carbon atoms in the molecule. The tens place, the 0, represents the number of hydrogen atoms in the molecule, and the ones place represents the number of fluorine atoms in the molecule. According to the rule of 90, CFC-12 has 1 carbon atom, 0 hydrogen atoms, and 2 fluorine atoms. The final aspect of the rule of 90 pertains to the necessity of carbon to form four bonds. If, after assigning hydrogen and fluorine atoms to it, the carbon atom still requires more bonds to reach the required four, those bonds will be to chlorine atoms. So far, the one carbon atom of CFC-12 has only formed two bonds, both with fluorine atoms. Thus the remaining two bonds go to chlorine atoms. [Science, v242; 666-668; 1988.] [Science, v249; 31-35; 1990.]