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400 BCE
Democritus
Democritus was born around 460 BCE in ancient Greece and died around 370 BCE. He was the first to propose an atomic theory of the universe. He proposed that dividing objects infinitely would have to eventually lead to a small enough piece that would be indivisible, and this led him to the idea of the atom. His ideas were unpopular at the time and wouldn’t be accepted for over 2000 years. -
Antoine Lavoisier
Antoine Lavoisier was born in France in 1743 and died there in 1794. He defined the Law of Conservation of Mass, which states that matter can never be created or destroyed, by studying many chemical reactions to determine whether either ever occurred. He was executed by guillotine during the Reign of Terror of the French Revolution as a result of his wealthy status. -
John Dalton
John Dalton was born in 1766 in England and died there in 1844. His atomic theory stated that atoms were different in size and mass in different elements, which he discovered by measuring the atomic mass of different elements. He is known as one of the “fathers of chemistry” for his discoveries. -
J.J. Thomson
J.J. Thomson was born in England in 1856 and died there in 1940. He was the first to discover the electron, doing so using cathode-ray tubes to determine that the rays were composed of negatively-charged particles. He was a teacher at many institutions throughout his career and was head of the Cavendish Laboratory. -
Max Planck
Max Planck was born in Germany in 1858 and died there in 1947. He discovered quanta, which are packets of energy, as a result of the phenomenon of light intensity depending on wavelength. Quanta would later be used by Bohr and other scientists to develop modern atomic theory. Planck won the Nobel Prize for Physics in 1918. -
Ernest Rutherford
Ernest Rutherford was born in New Zealand in 1871 and died in England in 1937. He discovered the nucleus and the proton of the atom. He used the gold foil experiment to discover the nucleus, which involved shooting particles at an atom, and the proton by disintegrating an atom which gave off a positive particle. He was president of the Academic Assistance Council in 1933 which helped German refugees from the Nazis. -
Niels Bohr
Niels Bohr was born in 1885 in Denmark and died there in 1962. He proposed a model which had electrons at fixed orbits around the nucleus and at fixed energy levels, and had electrons moving to further levels from the nucleus if energy was gained and closer levels if energy was lost. He discovered this model by applying Planck’s idea of quanta to explain how electrons could have stable orbits around the nucleus. He organized the Atoms for Peace Conference in 1955. -
James Chadwick
James Chadwick was born in England in 1891 and died there in 1974. He discovered the neutron by performing an experiment with beryllium being shot with alpha particles that released radiation which had particles similar to a proton’s mass but uncharged. He won for the Nobel Prize for Physics in 1935 for this discovery.