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460 BCE
Democritus
One of the first atomic theorists was Democritus, a Greek philosopher who lived in the fifth century BC. Democritus knew that if a stone was divided in half, the two halves would have essentially the same properties as the whole.Therefore, he reasoned that if the stone were to be continually cut into smaller and smaller pieces then; at some point, there would be a piece which would be so small as to be indivisible. He called these small pieces of matter "atomos," the Greek word for indivisible. -
322 BCE
Aristotle
Aristotle did not believe in the atomic theory and he taught so otherwise. He 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. Aristotle's view was finally proven incorrect and his teachings are not present in the modern view of the atom. -
Isaac Newton
Isaac Newton theorized a mechanical universe with small, solid masses in motion. -
John Dalton
John Dalton proposed that elements consisted of atoms that were identical and had the same mass and that compounds were atoms from different elements combined together. -
Michael Faraday
Michael Faraday developed the two laws of electrochemistry. -
J. Plucker
built one of the first cathode-ray tubes. -
J. Plucker
built one of the first cathode-ray tubes. -
Dmitri Mendeleev
created the periodic table. -
Dmitri Mendeleev
created the periodic table -
James Clerk Maxwell
proposed the theory of electromagnetism and made the connection between light and electromagnetic waves. -
James Clerk Maxwell
proposed the theory of electromagnetism and made the connection between light and electromagnetic waves. -
G.J. Stoney
theorized that electricity was comprised of negative particles he called electrons. -
G.J. Stoney
theorized that electricity was comprised of negative particles he called electrons. -
Sir William Crookes
experiments with cathode-ray tubes led him to confirm the work of earlier scientists by definitively demonstrating that cathode-rays have a negative charge. -
Sir William Crookes
experiments with cathode-ray tubes led him to confirm the work of earlier scientists by definitively demonstrating that cathode-rays have a negative charge. -
E. Goldstein
discovered canal rays, which have a positive charge equal to an electron. -
E. Goldstein
discovered canal rays, which have a positive charge equal to an electron. -
Wilhelm Roentgen
discovered x-rays. -
Wilhelm Roentgen
discovered x-rays. -
Henri Becquerel
discovered radiation by studying the effects of x-rays on photographic film. -
Henri Becquerel
discovered radiation by studying the effects of x-rays on photographic film. -
J.J. Thomson
determined the charge to mass ratio of electrons. -
J.J. Thomson
determined the charge to mass ratio of electrons. -
Rutherford
discovered alpha, beta, and gamma rays in radiation -
Rutherford
discovered alpha, beta, and gamma rays in radiation. -
Marie Sklodowska Curie
discovered radium and polonium and coined the term radioactivity after studying the decay process of uranium and thorium. -
Marie Sklodowska Curie
discovered radium and polonium and coined the term radioactivity after studying the decay process of uranium and thorium. -
Max Planck
proposed the idea of quantization to explain how a hot, glowing object emitted light. -
Frederick Soddy
came up with the term "isotope" to explain the unintentional breakdown of radioactive elements. -
Max Planck
proposed the idea of quantization to explain how a hot, glowing object emitted light. -
Frederick Soddy
came up with the term "isotope" to explain the unintentional breakdown of radioactive elements. -
Hantaro Nagaoka
proposed an atomic model called the Saturnian Model to describe the structure of an atom. -
Hantaro Nagaoka
proposed an atomic model called the Saturnian Model to describe the structure of an atom. -
Richard Abegg
found that inert gases have a “stable electron configuration.” -
Richard Abegg
found that inert gases have a “stable electron configuration.” -
Hans Geiger
invented a device that could detect alpha particles. -
Hans Geiger
invented a device that could detect alpha particles. -
H.G.J. Moseley
discovered that the number of protons in an element determines its atomic number. -
H.G.J. Moseley
discovered that the number of protons in an element determines its atomic number. -
Francis William Aston
used a mass spectrograph to identify 212 isotopes. -
Francis William Aston
used a mass spectrograph to identify 212 isotopes. -
Niels Bohr
proposed an atomic structure theory that stated the outer orbit of an atom could hold more electrons than the inner orbit. -
Niels Bohr
proposed an atomic structure theory that stated the outer orbit of an atom could hold more electrons than the inner orbit. -
Louis de Broglie
proposed that electrons have a wave/particle duality. -
Cockcroft / Walton
created the first nuclear reaction, producing alpha particles -
Paul Dirac
proposed the existence of anti-particles. -
James Chadwick
discovered neutrons, particles whose mass was close to that of a proton. -
Lise Meitner, Hahn, Strassman
discovered nuclear fission. -
Glenn Seaborg
discovered eight transuranium elements. -
Enrico Fermi
created the first man-made nuclear reactor.