Final answer:
The number of electrons is equal to the number of protons in an atom. The number of neutrons in an atom can be calculated by subtracting the atomic number from the mass number.
Step-by-step explanation:
An atom's mass number is calculated by adding the number of protons (atomic number) and the number of neutrons together. The number of electrons in an atom is equal to the number of protons. The formula for calculating the number of neutrons in an atom is mass number - atomic number. For example, if the mass number is 25 and the atomic number is 12, there would be 13 neutrons in the atom.