-
Period: 400 BCE to 450
Democritus
Democritus discovered the atomic theory around 400 to 450 B.C.E. Democritus developed his atomic philosophy as a middle ground between two opposing Greek theories about reality and the illusion of change. He discovered it around 400 B.C.E. His famous disciple, Democritus of Abdera, named the building blocks of matter atoms, meaning “indivisible,” about 430 BCE. -
Antoine Lavoisier
Antione Lavoisier made his discovery in 1783. Lavoisier discovered the role of oxygen in combustion and respiration, proved the law of conservation, reformed the chemical nomenclature, and named hydrogen. He developed the modern system of naming chemical substances and has been called the “father of modern chemistry” for his emphasis on careful experimentation. -
John Dalton
John Dalton made his discovery in 1803. An important development in Dalton's theory is that all matter is composed of extremely small particles called atoms. He introduced his belief that atoms of different elements could be universally distinguished based on their varying atomic weights. -
Eugene Goldstein
Eugene Goldstein made his discovery in 1886. Eugene Goldstein discovered positive particles by using a tube filled with hydrogen gas. The positive particle had a charge equal to and opposite to the electron. The positive particle was named the proton. -
Henri Becquerel
Henri Becquerel made his discovery in 1896. Becquerel discovered that uranium salts spontaneously emit penetrating radiation that can be registered on a photographic plate. In May he announced that the element uranium was indeed what was emitting the radiation. Becquerel initially believed his rays were similar to x-rays, but his further experiments showed that unlike x-rays, which are neutral, his rays could be deflected by electric or magnetic fields. -
J.J. Thomson
In 1897 Thomson discovered the electron. He also discovered the atom in 1897. He discovered that all atoms contain tiny negatively charged subatomic particles or electrons. Thomson's work also led to the invention of the mass spectrograph. Thomson discovered the plum pudding model of the atom, which had negatively-charged electrons . -
Max Planck
Max Planck discovered the atom in 1900. He introduced the theory of "quanta". Quanta radiation consists of quanta with specific energies determined by a new fundamental constant, thereafter called Planck's constant. This theory revolutionized our understanding of atomic and subatomic processes, just as Albert Einstein's theory of relativity revolutionized our understanding of space and time. Planck demonstrated that energy, in certain situations, can exhibit characteristics of physical matter. -
Robert Millikan
Robert Millikan made his discovery in 1910. Millikan discovered the elementary charge of an electron using the oil-drop experiment. He discovered that there is a fundamental electric charge in electrons. Robert Millikan charged droplets of oil between two electrodes and balanced the gravitational force with the upward forces, using mechanical equilibrium to determine the charge, which he found within one percent of the currently accepted value. -
Ernest Rutherford
Ernest Rutherford made his discovery of the atomic theory in 1911. He is known for his pioneering studies of radioactivity and the atom. Rutherford postulated the nuclear structure of the atom. Rutherford's experiment showed the existence of a nuclear atom - a small, positively-charged nucleus surrounded by empty space and then a layer of electrons to form the outside of the atom. -
Niels Bohr
Niels Bohr made his discovery in 1913. Niels Bohr discovered a theory for the hydrogen atom. Bohr was the first to discover that electrons travel in separate orbits around the nucleus and that the number of electrons in the outer orbit determines the properties of an element. Bohr changed the atomic theory by realizing that the electrons did not crash into the nucleus as would be expected in classical physics. The Bohr model postulates that electrons orbit the nucleus at fixed energy levels. -
Louis de Broglie
Louis de Broglie made his discovery in 1924. Broglie discovered the idea that particles, such as electrons, could be described not only as particles but also as waves. He postulated the wave nature of electrons and suggested that all matter has wave properties. Broglie states that matter can act as waves much like light and radiation, which also behave as waves and particles. The equation further explains that a beam of electrons can also be diffracted just like a beam of light. -
Erwin Schrodinger
Erwin Schrodinger made his discovery in 1926. Erwin Schrodinger discovered wave equations that accurately calculated the energy levels of electrons in atoms.
Erwin Schrödinger showed that the quantization of the hydrogen atom's energy levels that appeared in Niels Bohr's atomic model could be calculated from the Schrödinger equation, which describes how the wave function of a quantum mechanical system evolves. -
Werner Heisenberg
Werner Heisenberg made his discovery in 1927. formulating quantum mechanics in terms of matrices and in discovering the uncertainty principle, which states that a particle's position and momentum cannot both be known exactly. Heisenberg formulated quantum mechanics in terms of matrices and in discovering the uncertainty principle, which states that a particle's position and momentum cannot both be known exactly. -
James Chadwick
James Chadwick made his discovery in 1932. Chadwick discovered the existence of neutrons. Chadwick fired alpha radiation at a beryllium sheet from a polonium source. This led to the production of uncharged, penetrating radiation. This radiation was made incident on paraffin wax, a hydrocarbon having a relatively high hydrogen content. This is how Neutrons were discovered. Chadwick's discovery forced a revision of the cloud model.