In nuclear physics, isotones are nucleides of the different chemical elements. They have the same neutron number, but different nucleon number (mass number) due to different proton number (atomic number). For example, boron-12 and carbon-13 nuclei both contain 7 neutrons, and so are isotones. Similarly, S, Cl, Ar, K, and Ca nuclei are all isotones of 20 because they all contain 20 neutrons.