(a) All nuclei are composed of two types of particles protons and neutrons.
(b) The only exception is the ordinary hydrogen nucleus which is just a single proton.
(c) A proton has a mass of 1.6726 × 10–27 kg and charge +e (= 1.6 × 10–19 C).
(d) A neutron has a mass of 1.6750 × 10–27 kg and is electrically neutral.
The atomic number Z: This is equal to the number of protons in the nucleus.
The neutron number N: This is equal to the number of neutrons in the nucleus.
The mass number A: This is equal to the number of nucleons(protons + neutrons) in the nucleus.
Thus, A = Z + N
Symbolically a nucleus X shall be represented as zXA .
