There are two forces operating inside the nucleus of an atom : (i) the electrostatic force which causes the repulsion between various protons and tends to make the nucleus unstable, and (ii) the nuclear force or strong force which causes attraction between protons and protons, protons and neutrons, and neutrons and neutrons, and tends to make the nucleus stable.
Now, if an atom is small, then its nucleus is also small. In a small nucleus, the distances between nuclear particles (protons and neutrons) are very, very small due to which the nuclear force of attraction is very large as compared to the electrostatic force of repulsion. Since the nuclear force of attraction is much greater than the electrostatic force of repulsion, so a small nucleus is very stable.
Thus, despite the repulsion between positively charged protons, the nuclei of small atoms are stable because of the presence of a strong nuclear force (or strong force) of attraction in the nucleus. Since the nuclei of most of the atoms are small in which the protons and neutrons are very, very close together, it is the nuclear force of attraction which is greater (than the electrostatic force of repulsion), and makes the nuclei very stable. We shall now discuss why big atoms or big nuclei are unstable.
If an atom is big, then its nucleus will also be quite big. Now, in a big nucleus, the distances between the nuclear particles (protons and neutrons) are comparatively large due to which the nuclear force of attraction or strong force of attraction between them becomes weak. It should be noted that in a big nucleus, the electrostatic force of repulsion also decreases but this decrease is much less as compared to the decrease in nuclear force of attraction.
So, in a big nucleus, the electrostatic force of repulsion becomes slightly greater than the nuclear force of attraction because of which the big nucleus becomes unstable. From the above discussion we conclude that it is the atoms having large nucleus (having a large mass number) which are unstable. The uranium-235 atom is one such big atom having a large nucleus of mass number 235, which is unstable (235 is the total number of protons and neutrons in the nucleus of this uranium atom. It includes 92 protons and 143 neutrons).
We shall now discuss why a uranium-235 atom or uranium-235 nucleus is unstable. The uranium-235 atom is quite big in size, so its nucleus is also big. Due to the big size of the uranium-235 nucleus, the distances between various protons and neutrons in it are comparatively large, and this makes the nuclear force of attraction weak. So, in uranium-235 nucleus, the electrostatic force of repulsion between protons becomes slightly greater than the nuclear force of attraction (or strong force of attraction). And this makes the uranium-235 nucleus unstable. Because of this unstable nucleus, uranium-235 shows radioactivity by emitting certain particles and radiations spontaneously.
Actually, there is a very delicate balance between the electrostatic force and nuclear force in a uranium-235 nucleus. And when this delicate balance is disturbed, then the whole uranium-235 nucleus breaks up or disintegrates. For example, when even a slow moving neutron hits a uranium-235 nucleus, the balance of forces in the nucleus is upset, causing the fission of uranium nucleus to form smaller nuclei.