It is a known fact that from the scattering of α – particles, scientist Rutherford concluded that the atom of any element consists of central core called nucleus and electrons moving around it. The entire mass of the atom and positive charge is concentrated inside the nucleus. The nucleus is supposed to consist of two particles, the proton and the neutron. Their masses are nearly the same as :
Mass of the proton = 1.67261 x 10-27kg
Mass of the neutron = 1.67492 x 10-27kg
The proton is a positively charged particle while the neutron is a neutral particle, that is, it carries no charge. The charge on the proton is 1.6 x 10-19 coulomb. Both the proton and neutron together in the nucleus are called nucleons . The number of protons in the nucleus is called the atomic number and the sum of protons and neutrons is called as mass number. The stability of a nucleus depends on the relative number of protons and neutrons.
The conventional symbols for nuclear species follow the following pattern
z X A
where X = Chemical symbol of the element
Z = Atomic number(i.e., number of protons in the nucleus)
and A = Mass number (i.e., number of protons and neutron in the nucleus)
Therefore, the number of neutrons in the nucleus
N = A – Z
For example, 15P31 has Z = 15 and A = 31
Therefore, the number of neutrons N =31 – 15 = 16
Number of electrons = 15
Number of protons = 15
The electrons move or one should say spin and revolve around nucleus like earth around sun. The electron stays in shells or orbits. The mass of the electron (9.108 x 10-31 kg) is very very small.