There are two elements—calcium (atomic number 20) and argon (atomic number 18). The mass number of both the elements is 40. They are therefore known as

The elements calcium (atomic number 20) and argon (atomic number 18) both have a mass number of 40. Atoms of different elements that possess the same mass number but different atomic numbers are defined as isobars. While isotopes share the same atomic number but differ in mass number, isobars represent different chemical elements because they have different numbers of protons. In this specific case, calcium has 20 protons and 20 neutrons, whereas argon has 18 protons and 22 neutrons, yet both sum to a total nucleon count (mass number) of 40. Consequently, they exhibit different chemical properties but share the same total mass. Other terms like isotones refer to atoms with the same number of neutrons, and isotopes refer to atoms of the same element with different masses.