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460 BCE
Democritus
Democritus suggested that all matter is made of different types of tiny particles, and that the properties of the particles determined the properties of the matter. -
Antoine Lavoisier
Lavoisier found that matter is neither created or destroyed in a chemical reaction, or more specifically, the amount of matter at the start of a reaction will equal the amount of matter at the end of a reaction. This observation later became the Law of Conservation of Matter. -
John Dalton
Dalton developed the first modern atomic theory, with spherical solid atoms based upon measurable properties of mass. This theory stated that all atoms of the same element are identical, the atoms of different elements vary in size and mass, and that chemical reactions have no bearing on the creation or destruction of atoms; they can only rearrange them. The theory also said that all matter is made of different combinations of atoms. -
Marie Curie
Curie studied radioactivity while observing the decay process of uranium and thorium, and then used her discoveries to explore the use of radioactivity in medical applications. -
Albert Einstein
Einstein was able to actually prove that atoms exist, using mathematical laws and equations. -
J.J. Thompson
Thompson discovered that cathode rays hold a negative magnetic charge, and that the particle that make up the stream, called electrons, are smaller than the smallest known atom. This proved that atoms themselves are not the basic building blocks of matter, but that the subatomic particles (electrons) are. From this he developed the Plum Pudding Model, which shows a positively charged sphere containing an array of negatively charged particles. -
Ernest Rutherford
Rutherford discovered and named some of the most basic principles in physics, such as alpha, beta, and gamma rays, as well as the proton, neutron, half-life, and daughter atoms. He also developed a model of the atom that resembled the solar system, with electrons orbiting around a nucleus like planets around the sun. -
Max Planck
Planck discovered that energy is not radiated in packets, but in an unbroken wave. -
Niels Bohr
Bohr concluded that electrons are found at fixed distances from the nucleus in specific orbitals. If energy is added to the atom, electrons move to the next energy level away from the nucleus; if energy is lost, they move an energy level toward the nucleus. -
Robert Millikan
Millikan was able to calculate the specific negative charge of the electron. -
Werner Heisenburg
He developed an equation called the Uncertainty Principle, which states that electrons do not travel in neat orbits. He also developed matrix mechanics, a mathematical system to atomic physics that explains electrons in an atom. -
Louis de Broglie
De Broglie proposed that electrons have the properties of a wave, therefore explaining the already proven fact that electrons move around the nucleus in a restricted motion. His proposition said that a wave with boundaries (due to the nuclear charge) would be restricted in shape and furthermore, in motion. -
Erwin Schrodinger
Schrodinger developed the Quantum Mechanical Model, which describes the likelihood of finding electrons in a certain location. An example of this would be an electron cloud that surrounds the nucleus, with the places where the cloud is most dense being the places where it is most likely to find an electron. It was this information that influenced the concept of sub-energy levels. -
James Chadwick
Chadwick discovered the neutron, which holds the neutral electrical charge in an atom.
