To calculate the number of neutrons in an atom, what must you do?

To determine the number of neutrons in an atom, you take the atomic mass and subtract the number of protons. The atomic mass, typically expressed as a whole number, represents the total number of protons and neutrons in the nucleus of an atom. Since the atomic number indicates how many protons are present, subtracting this value from the atomic mass gives you the number of neutrons.

For example, if an atom has an atomic mass of 12 and has 6 protons (which is the atomic number for carbon), the calculation would be 12 (atomic mass) - 6 (protons) = 6 neutrons. This method accurately reflects the balance of particles in the atom's nucleus.

Other methods presented in the options, such as counting electrons or adding protons to atomic mass, do not yield the correct number of neutrons, as electrons do not contribute to the atomic mass and adding protons would count them twice. Doubling the number of protons also doesn't relate to finding the neutrons. Thus, the correct approach to find the number of neutrons is to subtract the number of protons from the atomic mass.