Because protons and neutrons have a mass of 1 amu each as known earlier, the mass number equals the sum of the numbers of protons and neutrons. As the mass of the electron was found to be very small, it was also suggested that the entire mass of the atom must comprise of positively charged particles.
Electrons have a mass of approximately 1/1836 amu and have a negative charge. Electrons have 1/1836 the mass of protons and a negative charge assigned at -1 unit since the electron is the basis of all charges.
The third type of particle is found orbiting the nucleus – the electron. Every atom has the same number of electrons orbiting its nucleus as it has protons in its nucleus. The mass of a proton and a neutron are the same.