A radioactive nuclide that is used to label blood platelets has 49 protons and 62 neutrons. Which is the symbol of this nuclide?

The mass number is the total number of protons and neutrons: 49 + 62 = 111. In isotope notation, the mass number goes above the element symbol and the atomic number (49) goes below it. Therefore, the nuclide is indium-111, choice A.

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