Relative Isotopic Mass

The relative isotopic mass is the mass of an atom of an isotope compared to 1/12^th the mass of a C-12 atom.More precisely, the relative isotopic mass is the mass of an atom of an isotope relative to 1/12 of the mass of an atom of carbon-12.Carbon-12 is the standard. By definition an atom of carbon-12 has a relative isotopic mass of exactly 12, so 1/12 of its mass is 1 unit, roughly the mass of one proton or one neutron.relative isotopic mass = mass of one atom of the isotope ÷ (1/12 × mass of one atom of carbon-12)Relative isotopic mass is a ratio of two masses, so it has no units.Protons and neutrons each have a relative mass of about 1 and electrons have almost no mass, so the relative isotopic mass is very close to the mass number. For example, the relative isotopic mass of chlorine-35 is 34.97, which is usually rounded to 35. Unless you are given a more precise value, you can take the relative isotopic mass as equal to the mass number.Each isotope of an element has its own relative isotopic mass, so chlorine has two: 35 and 37.Worked example: one atom of an isotope of magnesium has twice the mass of an atom of carbon-12. Its relative isotopic mass is 2 × 12 = 24.