Our 24-hour Chem Time Clock replaces the usual numbers found on the face of a clock with symbols of elements having corresponding atomic numbers. 'H' replaces '1', 'He' replaces '2', etc. The idea for this clock came from Prof. Bassam Shakhashiri at the University of Wisconsin. It is the perfect addition to a science classroom. The face of the clock has black lettering and the outside trim is forest green. Diameter is 13". AA battery is required and available separately.