The number of electrons in a neutral atom is equal to the number of protons. The mass number of the atom (M) is equal to the sum of the number of protons and neutrons in the nucleus. The number of neutrons is equal to the difference between the mass number of the atom (M) and the atomic number (Z).
Protons, neutrons, and electrons: Both protons and neutrons have a mass of 1 amu and are found in the nucleus. However, protons have a charge of +1, and neutrons are uncharged. Electrons have a mass of approximately 0 amu, orbit the nucleus, and have a charge of -1.
If you add or subtract a proton from the nucleus, you create a new element. If you add or subtract a neutron from the nucleus, you create a new isotope of the same element you started with. In a neutral atom, the number of positively charged protons in the nucleus is equal to the number of orbiting electrons.