Naturally occurring chlorine is composed of two isotopes: 75.76% Cl-35 (mass 34.9688 amu) and 24.24% Cl-37 (mass 36.9659 amu). Naturally occurring oxygen is composed of three isotopes: 99.757% O-16 (mass 15.9949 amu); 0.038% O-17 (mass 16.9991 amu); and 0.205% O-18 (mass 17.9991 amu). The compound dichloromonoxide is composed of two chlorine atoms and one oxygen atom bonded together to form the molecule.