Atomic Theory Timeline

  • 400 BCE

    Democritus's Theory of Atoms

    Democritus, a philosopher in ancient Greece,
    began the search for a description of matter.
    He questioned whether matter could be
    divided into smaller and smaller pieces
    forever until eventually the smallest possible
    piece would be obtained. He believed that
    the smallest possible piece of matter was
    indivisible. He named the smallest piece of
    matter “atomos,” meaning “not to be cut.”
  • Period: 300 BCE to

    Aristotle's Atom Theory

    In ancient Greece, the popular
    philosopher Aristotle declared
    that all matter was made of only
    four elements: fire, air, water
    and earth. He also believed that
    matter had just four properties:
    hot, cold, dry and wet.
  • Dalton's Atom Theory

    In the early 1800s, the English Chemist John
    Dalton performed a number of experiments
    that eventually led to the acceptance of the
    idea of atoms. He formulated the first atomic
    theory since the “death of chemistry” that
    occurred during the prior 2000 years.
    Dalton theorized that all matter is made of
    atoms. Atoms are too small to see,
    “uncuttable,” and indestructible. All atoms of
    a given element are exactly alike and atoms
    of different elements are different.
  • Thompson's Atom Theory

    In 1897, the English scientist named J.J.
    Thomson provided the first hint that an
    atom is made of even smaller particles.
    He discovered the presence of a negative
    particle in the atom – the electron.
    He proposed a model of the atom that is
    sometimes called the “Plum Pudding” model.
    His theory was that atoms are made from a
    positively charged substance with negatively
    charged electrons scattered about, like
    raisins in a pudding or chocolate chips in a
  • Einstein's Atom Theory

    Einstein also in 1905 mathematically proved the existence of atoms, and thus helped revolutionize all the sciences through the use of statistics and probability. Atomic theory says that any liquid is made up of molecules (invisible in 1905). Furthermore, these molecules are always in random, ceaseless motion.
  • Rutherford's Atom Theory

    In 1908, the English physicist Ernest
    Rutherford performed an experiment using
    positively charged particles fired at gold foil.
    Through his experiment, he proved that
    atoms are not a “pudding” filled with a
    positively charged material. He theorized
    that atoms have a small, dense, positively
    charged center, which he called the
    “nucleus”. He said that nucleus is tiny
    compared to the atom as a whole, because
    the atom is mostly open space!
  • Bohr's Atom Theory

    In 1913, the Danish scientist Niels Bohr
    proposed an improvement. He built on the
    concept that the mass of an atom is
    contained mostly in the nucleus. He also
    theorized that electrons move in definite
    orbits around the nucleus, much like planets
    circle the sun. These orbits, or energy levels,
    are located at certain distances from the
  • Shrodinger's Atom Theory

    Based on de Broglie's idea that particles could exhibit wavelike behavior, Austrian physicist Erwin Schrödinger theorized that the behavior of electrons within atoms could be explained by treating them mathematically as matter waves.
  • Heisenberg's Atom Theory

    Werner Heisenberg contributed to atomic theory through 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.
  • Period: to

    Modern Cloud Theory

    According to today’s atomic
    theory, electrons do not orbit
    the nucleus in neat planet-like
    orbits but move at high speeds
    in an electron cloud around the
    In the electron cloud, electrons
    whirl around the nucleus
    billions of times in one second.
    They are not moving around in
    random patterns; an electron’s
    location depends upon how
    much energy the electron has.