As discussed in the previous chapter, according to Dalton’s atomic theory, atoms are the building blocks of matter. In the year 1808, John Dalton proposed atomic theory saying that atom is an indivisible particle of matter. By the end of the nineteenth century, experiments proved that atoms comprise of three particles viz protons, neutrons, and electrons. Therefore, the discovery of sub-atomic particles gave rise to various atomic models to explain the structure of an atom.