7.9 g (I suppose grams)/45 = moles Sc.
6.022 x 10^23 atoms Sc/1 mol. Calculate number of atoms Sc you have.
# atoms Sc x mass 21 electrons.
# atoms Sc x mass 21 protons.
# atoms Sc x mass (45-21) neutrons.
total mass = mass e + mass p + mass n
calculate the number of protons, neutrons, and electrons in the sample of scandium that you weighed at 7.9 mass weight if it is 100% ^45Sc.
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