Ernest Rutherford (1871-1937) was a New Zealand chemist and physicist; he is considered by many as the founder of nuclear physics.
He posited the thesis that the electron orbits around the nucleus of the atom (the nucleus consists of the proton and neutron). For this discovery he was awarded the Nobel Prize in Chemistry in 1908.
Rutherford discovered that the atom has a positively charged nucleus orbited by electrons.
He showed that atoms do spontaneously disintegrate (decay) and do so at given rates, that is, took the same amount of time for that to happen (half time); this discovery gave scientists a better way to determine the age of the earth (which turned from six thousand years to four and half billion years).
Rutherford studied the transmutation of one element to another and in 1919 became the first human being to transmute one element, oxygen, into another, nitrogen; he did so through nuclear reaction.
Rutherford, working with Neils Bohr in the 1920s postulated that the nucleus of the atom is composed not just of protons, as was then thought, but also of neutrons. Neutrons compensated for the repelling effect of the positive charges of protons by causing an attractive nuclear force and thus keeping the nuclei from breaking apart.
Ernst Rutherford dabbled in other aspects of Chemistry and physics but will forever remain known as the person who explained the structure and composition of the atom.
Ernst Rutherford. Radio-activity. (1904)
Ernest Rutherford. Radioactive Transformations. (1906)
Ernest Rutherford. Radiations from Radioactive Substances. (1919)
Ernst Rutherford. The Electrical Structure of matter. (1926)
Ernest Rutherford. The Artificial Transmutation of the Elements. (1933)