The study of chemistry began with John Dalton who first discovered the atom. The study of the atom ranged from 1766 to 1844. He is famously known for introducing the atomic theory into chemistry. He discovered that elements contains atoms, they are identical in an element and they cannot be created nor destroyed. The theory was not accurate and it required more research.

Later on, another chemist, JJ Thompson who continued the atomic theory had discovered the electrons. In 1897, he studied cathode rays which proved that atoms were not indivisible. Electrons consist of negative charges which are subatomic particles. This theory was inaccurate and more research was needed.

The third chemist named "Ernest Rutherford" discovered particles and light rays in atoms from 1871 to 1937. These include alpha and beta. These light particles are considered radioactive and they decay which is a process called radioactive decay. He identified that alpha particles are equivalent to helium nuclei. The theory was still unclear. A lot more research is required.

The last chemist who contributed to the atomic theory was Niels Henrick David Bohr, (formally Niels Bohr.) He won the Nobel Prize for proposing the research of valence electrons of an atom in 1922. The atom contained electrons in an energy shell where the innermost had the least amount of energy and the outermost had the most amount of energy. The theory is understandable and accurate. Thus, we are still using his model today.