Doubt & Answer Explanation

Answer Explanation

Neutron to proton ratio is the ratio generally used and it refers to the number of neutrons/number of protons in a nucleus.

Neutrons and protons inside the nucleus attract each other because of short range nuclear forces. These forces are active when distances between two particles are less than 10^-12 m. So, even though two protons are positively charged, they attract each other inside the nucleus because of the nuclear forces which are stronger than the electrical forces between them.

When the number of protons increases, then the electrical coulomb forces between them also become significant and the nucleus starts becoming unstable. However, if there is a large number of neutrons also present, then since the neutrons will only have attractive forces between them (and NO repulsive forces), the average repulsive forces per nucleon will decrease because of the presence of neutrons.  This does not however mean that the larger the number of neutrons - the higher the stability of the nucleus. In fact after using a lot of concepts that are not relevant at this level of study, scientists have come up with a line called the stability line which indicates the exact ration between neutrons and protons so that the nucleus is stable. If the number of neutrons is much higher or lesser than mentioned by the stability line, then the nucleus becomes unstable.