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Question:

An atom of bismuth has a mass number of 209 and an atomic number of 83. How many neutrons are in one bismuth atom?

A 126
Explaination

To find the number of neutrons, use the following formula: Mass Number = Number of Protons + Number of Neutrons. The atomic number equals the number of protons in an atom of an element. Thus, 209 = 83 + Number of Neutrons or 209 \u2013 83 = 126.