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400 BCE
Democritus
Proposed that matter couldn't be divided into smaller pieces forever. Also claimed matter was made up of small, "hard" particles called atoms. -
340 BCE
Aristotle
Didn't believe in the atomic theory, actually taught different. thought that all materials on Earth were not made of atoms, but of the four elements, Earth, Fire, Water, and Air. He believed all substances were made of small amounts of these four elements of matter. -
Lavoisier
He wrote the first main list of elements. He also had discovered the role oxygen plays in combustion. He recognized and named oxygen (during 1778) and hydrogen (during 1783), and opposed the phlogiston -
Dalton
Created the very first atomic theory. He viewed atoms as tiny solid balls. -
Proust
Proust discovered what is today called "Proust's law". This law
stated that chemical compounds are formed of constant and defined ratios of elements, as determined by mass. -
Faraday
Faraday made one of the most significant discoveries which led to the idea of atoms having an electrical component. The particles that were first believed to be negatively charged atoms or molecules -
Becquerel
Becquerel discovered the radioactivity in uranium. This caused scientists to change their ideas about atomic structure drastically. -
Thomson
He made the atomic model known as, "the raisin bun model". He showed the atom was made up of even smaller things. -
Planck
He invented the quantum theory. -
Marie & Pierre Curie
They had, "successfully isolate radioactive radium salts from the mineral pitchblende in their laboratory in Paris." -
Einstein
Einstein discovered that energy and mass are interchangeable, setting the stage for nuclear power and atomic weapons. E = MC^2 -
Millikan
Millikan began a series of experiments to determine the electric charge carried by a single electron. He ended up discovering that all of the drops had charges that were simple multiples of a single number, the fundamental charge of the electron. -
Rutherford
Discovered protons and the nucleus. He also showed that atoms have (+) particles in the center, and are mostly empty space. -
Bohr
Proposed that electrons move around the nucleus, in specific layers. Also improved Rutherford's model. -
Chadwick
Worked with Rutherford, discovered particles have no charge. The discovery that resulted from this was the discovery of neutrons. -
Modern Day
New modern day atomic model has electrons moving around the nucleus in a cloud. Work that has been done since 1920 has changed the model.
