The word isotope refers to forms of the same element having different atomic weights. For example, there are five isotopes of ordinary iron in nature, and a preparation of iron used therapeutically contains these different isotopes in the following percentages:
All have the same nuclear charge, which means that they are iron, but each has a different total atomic weight. Similarly, other elements have different isotopic forms. For instance, there is hydrogen, with atomic weight 1; and heavy hydrogen, with atomic weight 2, which is twice as heavy. Hydrogen in nature contains 99.98 per cent H1 and 0.02 per cent H2. Other examples are nitrogen, with two isotopes, N14 (99.62 per cent), and N15 (0.38 per cent) and carbon, with two isotopes C12 (98.9 per cent) and C13 (1.1 per cent). In a given chemical or biologic sample, the percentage of these isotopes in
LAWRENCE JH. THE USE OF ISOTOPES IN MEDICAL RESEARCH. JAMA. 1947;134(3):219–225. doi:10.1001/jama.1947.02880200001001
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