The mass number of Fo? is 56. How many neutrons are there in a single Fo? atom? 28
30
56
58

 A. 28

If we aren't talking about an isotope, then the number of neutrons and the number of protons in the nucleus are the same. Each proton weighs one atomic unit, and the same can be said about a neutron as well. So half of the weight is from the protons and half is from the neutrons. The electrons are extremely small relative to the protons that their weight is negligible.

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