Is the nucleus a solid body? Is it an elementary building block of nature?! Although it is very small, a nucleus consists of something even smaller.
The only way to do experiments with such small objects as atoms and nuclei, is to collide them with each other and watch what happens. Perhaps you think that this is a barbaric way, like colliding a Mercedes and Toyota in order to learn what is under their bonnets. But with microscopic particles nothing else can be done.
In the early 1920s Rutherford and other physicists made many experiments, changing one element into another by striking them with energetic helium nuclei. They noticed that all the time hydrogen nuclei were emitted in the process. It was apparent that the hydrogen nucleus played a fundamental role in nuclear structure and was a constituent part of all other nuclei. By the late 1920's physicists were regularly referring to hydrogen nucleus as proton. The term proton seems to have been coined by Rutherford, and first appears in print in 1920.
Thus it was established that atomic nuclei consist of protons. The number of protons in a nucleus is such that makes up its positive charge. This number, therefore, coincides with the atomic number of the element in the Mendeleev's Periodic table.
This sounded nice and logical, but serious questions remained. Indeed, how can positively charged protons stay together in a nucleus? Repelling each other by electric force, they should fly away in different directions. Who keeps them together?
Furthermore, the proton mass is not enough to account for the nuclear masses. For example, if the protons were the only particles in the nucleus, then a helium nucleus (atomic number 2) would have two protons and therefore only twice the mass of hydrogen. However, it actually is four times heavier than hydrogen. This suggests that it must be something else inside nuclei in addition to protons.
These additional particles that kind of glue the protons and make up the nuclear mass, apparently, are electrically neutral. They were therefore called neutrons. Rutherford predicted the existence of the neutron in 1920. Twelve years later, in 1932, his assistant James Chadwick found it and measured its mass, which turned out to be almost the same but slightly larger than that of the proton.
Thus, in the early 1930s it was finally proved that atomic nucleus consists of two types of particles, the protons and neutrons. The protons are positively charged while the neutrons are electrically neutral. The proton charge is exactly equal but opposite to that of the electron. The masses of proton and neutron are almost the same, approximately 1836 and 1839 electron masses, respectively.
Apart from the electric charge, the proton and neutron have almost the same properties. This is why there is a common name for them: nucleon. Both the proton and neutron are nucleons, like a man and a woman are both humans. In physics literature, the proton is denoted by letter and the neutron by . Sometimes, when the difference between them is unimportant, it is used the letter meaning nucleon (in the same sense as using the word person instead of man or woman).
Chemical properties of an element are determined by the charge of its atomic nucleus, i.e. by the number of protons. This number is called the atomic number and is denoted by letter . The mass of an atom depends on how many nucleons its nucleus contains. The number of nucleons, i.e. total number of protons and neutrons, is called the atomic mass number and is denoted by letter .
Standard nuclear notation shows the chemical symbol, the mass number and the atomic number of the isotope.
For example, the iron nucleus (26-th place in the Mendeleev's periodic table of the elements) with 26 protons and 30 neutrons is denoted as , where the total nuclear charge is and the mass number . The number of neutrons is simply the difference (here, it is used the same letter , as for nucleon, but this should not cause any confusion). Chemical symbol is inseparably linked with . This is why the lower index is sometimes omitted and you may encounter the simplified notation like .
If we add or remove a few neutrons from a nucleus, the chemical properties of the atom remain the same because its charge is the same. This means that such atom should remain in the same place of the Periodic table. In Greek, same place reads (isos topos). The nuclei, having the same number of protons, but different number of neutrons, are called therefore isotopes.
Different isotopes of a given element have the same atomic number , but different mass numbers since they have different numbers of neutrons . Chemical properties of different isotopes of an element are identical, but they will often have great differences in nuclear stability. For stable isotopes of the light elements, the number of neutrons will be almost equal to the number of protons, but for heavier elements, the number of neutrons is always greater than and the neutron excess tends to grow when increases. This is because neutrons are kind of glue that keeps repelling protons together. The greater the repelling charge, the more glue you need.