1.4 The mass spectrometer

• Relative atomic masses are measured on a scale on which the mass of an atom of carbon-12 is defined as exactly 12. No other isotope has a relative atomic mass that is exactly a whole number because neither the proton nor the neutron has a mass of exactly 1.
• Chlorine has two isotopes. They are 35Cl with a mass number of 35 and 37Cl with a mass number of 37. They occur in the ratio of almost exactly 3:1.