The two isotopes of chlorine are 3517Cl and 3717Cl. Which isotope is the most abundant?
Since the number of neutrons varies in the periodic table, neither then number of neutrons nor the mass number (number of neutrons plus protons) is shown. What we do see is the atomic mass, or a weighted average of all the isotope masses. This is the noninteger listed with an element; it is the number with several decimal places. An element's atomic mass is the weighted average of the isotope masses. In other words, it is an average that takes into account the percentage of each isotope. To find the weighted average, multiply each isotopic mass by its relative abundance and find the sum for each isotope of an element.
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