What is a Nucleus
All elements are made up of atoms. The word atom comes from the Greek word atomos which means indivisible. The particles which make up atoms are protons neutrons and electrons. Protons and neutrons have equal mass, and together make up the mass of the nucleus. The electrons have very much lower mass than protons or neutrons, and when the mass of an atom is being considered, usually only the mass of the nucleus is taken into account. An atom is considered neutral because the number of protons found in the nucleus equals the number of electrons orbiting around it. The number of protons in the nucleus of an atom is called the atomic number of that element. It also equals the number of electrons in an atom, For an individual atom, the number of protons plus number of neutrons in the nucleus equals the mass number.
The atoms of some elements exist in different forms called isotopes which have different mass numbers. The total number of neutrons and protons in the nucleus of an atom determines the nuclide. The isotopes of an atom have more of neutron when compared to protons in the nucleus. For example, carbon atom has 6 protons and 6 neutrons in its nucleus; however, its isotope carbon-14 has 6 protons but 8 neutrons in its nucleus and is radioactive in nature. Thus, if number of neutron is more than the number of proton then the nucleus becomes unstable and results in radioactive decay.
The atoms of low atomic numbers which have different proton and neutron numbers in their nucleus can drop down to low energy state by radioactive decay to match the number of protons and neutrons in the nucleus. However, as the atomic number of the atom increases this becomes relatively difficult as the mutual repulsion between the protons requires an increasing proportion of neutrons in the nucleus. Thus the stability of all the elements with atomic number above 20 is reduced and there are no stable nuclei after Ca with atomic number 20. The requirement of neutrons per nuclei with increasing atomic number increases to about 1.5.
Nuclear fission is a process by which the number of protons and neutrons in the nucleus can be modified but the energy required for this is enormous. Atomic fission is defined as the energetic collision of two nuclei to give rise to heavier nuclei. Nuclear fission is an opposite process in which a nuclei splits into two smaller nuclei. The nucleus can also be modified by bombarding it with high energy subatomic particles or with proton. This process changes the atom to a different element.