As you know from earlier in this chapter, there are two kinds of particle in the nucleus of an atom:
protons, which carry positive charge $ + e$; and neutrons, which are uncharged. It is therefore quite surprising that the nucleus holds together at all. You would expect the electrostatic repulsions from all those positively charged protons to blow it apart. The fact that this does not happen is very good evidence for the existence of an attractive force between the nucleons. This is called the strong nuclear force. It only acts over very short distances (${10^{ - 14}}\,m$), and it is what holds the nucleus together.

Why are some atoms are unstable?

In small nuclei, the strong nuclear force from all the nucleons reaches most of the others in the nucleus, but as we go on adding protons and neutrons the balance becomes much finer. The longer-range electrostatic force affects the whole nucleus, but the short-range strong nuclear force of any particular nucleon only affects those nucleons around it – the rest of the nucleus is unaffected. In a large nucleus, the nucleons are not held together so tightly and this can make the nucleus unstable. The more protons there are in a nucleus, the greater the electric forces between them and we need a few extra neutrons to help ‘keep the protons apart’. This is why heavy nuclei have more neutrons than protons. The strong interaction can explain $\alpha  - $decay, but not $\beta  - $decay; we will look at this later in the chapter.
The proton and neutron numbers for some common nuclides are shown in Table 15.3. You can see that for light elements these two numbers are the same, but they become very different for heavy elements.
Adding more neutrons helps to keep the nucleus stable, but when the number of protons is greater than 83, adding more neutrons is not enough. Elements with a proton number greater than 83 are all unstable – they undergo radioactive decay.
Most atoms that make up our world have stable nuclei; that is, they do not change as time goes by, which is quite fortunate really! However, some are less stable and give out radiation. Whether or not an atom is unstable depends on the numbers of protons and neutrons in its nucleus. Hydrogen-1 $(1p)$, helium-4 $(2p,
2n)$, carbon-12 $(6p, 6n)$ and oxygen-16 $(8p, 8n)$ are all stable – but add or subtract neutrons and the situation changes.
For example, add a neutron to helium-4 and you get helium-5, a very unstable nucleus – it undergoes radioactive emission. (There is much more about radioactive decay later in this chapter.)