Out of these three particles, protons and neutrons located at the center of the atom as a hard and dense part known as nucleus. The rest of the part of atom contains negatively charged particles called as electron which balance out the charge of the protons and make the atom electrically neutral.
The total mass of an atom accumulate at the center of atom in the form of nucleus as the mass of electrons is negligible. Hence the sum of total number of protons and neutrons is called as mass number.
There must be some nuclear force which maintains the existence of nucleus, because there is a repulsion force between positively charged proton which are collected in a small region of nucleus. If the number of proton is less in an atom, other forces can hold the protons together and atom becomes stables. But as the ratio of protons to neutrons is increases, protons cannot be held firmly together and hence form an unstable nucleus.