An element is defined by the number of protons in the nucleus of the atom. For example, the element carbon has six protons, whereas the element oxygen has eight. Although the number of protons is fixed for an element, the number of neutrons can vary. Carbon can have six, seven, or eight neutrons. The various combinations of protons and neutrons are called isotopes, which are distinguished on the basis of atomic mass.
Atomic mass is the number of protons plus the number of neutrons.
Hence, they are called stable isotopes. S. Isotopic ratios are measured in the laboratory on an instrument known as a mass spectrometer. Before a sample can be analyzed for its isotopic ratio, the element of interest in the sample must be converted to a gaseous form. For hydrogen, carbon, oxygen, and sulfur isotopes, samples are converted to hydrogen gas, carbon dioxide, and sulfur dioxide, respectively. The extraction of gas from a rock or mineral typically involves a chemical reaction that destroys the rock or mineral.