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401 BCE
Democritus
Democritus was a Greek philosopher and one of the first people to discover atoms. he concluded that atoms were eindivisible and indestructible. Although Democritus’s ideas agreed with later scientific theory, they did not explain chemical behavior. His discovery did not have any form of experimental support because his approach was not based off the scientific method. -
John Dalton
Dalton's theory was that an element is composed of only one kind of atom, and a compound is composed of particles that are chemical combinations of different kinds of atoms. Chemical reactions occur when atoms are separated from each other, joined, or rearranged in a different combination. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction -
Eugen Goldstein
Eugen Goldstein observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays. He called these rays canal rays and figured out that they were composed of positive particles. Such positively charged subatomic particles are called protons. Each proton has a mass of about 1840 times that of an electron. -
J.J Thomson
J.J Thomson discovered electrons. Thomson performed experiments that involved passing an electric current through gases at low pressure. Thomson found that cathode rays are attracted to metal plates that have a positive electrical charge. -
Ernest Rutherford
Ernest Rutherford discovered a new theory of the atom. He suggested that the atom is mostly empty space, which explains the lack of deflection of most of the alpha particles. He concluded that all the positive charge and almost all the mass are condensed in a small region that has enough positive charge to account for the great deflection of some of the alpha particles. He then went on to call this region the nucleus. -
Robert A. Millikan
Robert Millikan was a U.S physicist that performed experiments to find out the quantity of an electron's charge. In his oil-drop experiment, Millikan suspended negatively charged oil droplets between two charged plates. He then changed the voltage on the plates to see how this affected the droplets’ rate of fall. -
James Chadwick
James Chadwick's discovery was the existence of neutrons. He discovered that neutrons have no charge. Atomic nuclei contain
neutrons and positively charged protons. His theory led to the discovery of the transmutation experiment. Neutrons were produced when beryllium-9 was combined with alpha particles.