Atomic Mass
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SAT Subject Test in Chemistry › Atomic Mass
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1
An atom of phosphorus has 15 protons, 16 neutrons, and 15 electrons. What is the atom's mass number?
Explanation
Protons and neutrons both have masses of approximately 1 amu, whereas the mass of an electron is negligible. Therefore, the mass number is equal to the number of protons and neutrons, which is 31.