Atomic Theory Timeline

  • 400 BCE


    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.”
  • 300 BCE


    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. Aristotle's influence and belief lasted until the early 1800's.
  • 250 BCE

    Alchemy and it's followers

    Alchemy and it's followers
    Alchemy is the art of understanding, deconstructing, and reconstruction of matter. It is composed of the 4 basic elements, Fire, Wind, Water, and Earth. Cerca 500 B.C, People began to decipher the composition of everyday objects. Alchemist's used many techniques to try and understand the matter that surrounded them and tried to explain it using patterns.
  • John Dalton

    John Dalton
    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.
  • J.J. Thomson

    J.J. Thomson
    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 cookie.
  • Max Planck

    Max Planck
    Max Planck was a German scientist who first discovered that energy is emitted from a black body in discrete amounts called quanta. He showed that the amount of energy was proportional to the frequency of the radiation that was absorbed by the black body, a relationship now known as Planck's Law.
  • Global Scientist (Sweden): Alfred Nobel

    Global Scientist (Sweden): Alfred Nobel
    Alfred Nobel was a Swedish chemist and inventor who invented dynamite. He was a prolific inventor and held 350 patents. He is perhaps most famous for starting the Nobel Prize. Alfred paved the way for scientists to be recognized properly for their incredible accomplishments. The element nobelium is named after Alfred Noble.
  • Global Scientist (Japan): Hantaro Nagaoka

    Global Scientist (Japan): Hantaro Nagaoka
    Nagaoka proposed an alternative planetary model of the atom in which a positively charged center is surrounded by a number of revolving electrons, in the manner of Saturn and its rings. Nagaoka's model made two predictions: a very massive atomic center and electrons revolving around the nucleus, bound by electrostatic forces.
  • Albert Einstein

    Albert Einstein
    Einstein’s 1905 paper on the matter/energy relationship proposed the equation E=MC2: the energy of a body (E) is equal to the mass (M) of that body times the speed of light squared (C2). This equation suggested that tiny particles of matter could be converted into huge amounts of energy, a discovery that heralded atomic power.
  • Ernest Rutherford

    Ernest Rutherford
    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 the nucleus is tiny compared to the atom as a whole because the atom is mostly open space. He concluded that electrons were scattered on the outside
  • Influential Woman: Marie Curie

    Influential Woman: Marie Curie
    Marie Curie discovered Radioactivity as well as both Polonium and Radium. She carried out extensive research with radioactivity and indirectly contributed to the constructions of the atomic bomb. She also pioneered the use of portable X-rays.
  • Niels Bohr

    Niels Bohr
    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 nucleus.
  • Louis de Broglie

    Louis de Broglie
    In his thesis, de Broglie developed his revolutionary theory of electron waves. The idea that matter on the atomic scale might have the properties of a wave was based on a proposal Einstein had made 20 years before. Einstein had suggested that light of short wavelengths might under some conditions be observed to behave as if it were composed of particles. The dual nature of light was just beginning to gain scientific acceptance when de Broglie extended the idea of such a duality to matter.
  • Wolfgang Pauli

    Wolfgang Pauli
    Pauli's most famous contribution was the now named "Pauli exclusion principle", which stated that no two electrons could inhabit the same quantum state. Pauli found that electrons could occupy the same orbital if they have opposite spin, or intrinsic angular momentum.
  • Erwin Schrödinger

    Erwin Schrödinger
    In 1926 Erwin Schrödinger, an Austrian physicist, took the Bohr atom model one step further. Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom. Unlike the Bohr model, the quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of the location of the electron.
  • Werner Heisenberg

    Werner Heisenberg
    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. The combined uncertainty in both measurements must be equal to or greater than h/(4π), where h is Planck’s constant.
  • James Chadwick

    James Chadwick
    In 1932, James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron.
  • Global Scientist (India) : Satyendra Nath Bose

    Global Scientist (India) : Satyendra Nath Bose
    Indian physicist Satyendra Nath Bose discovered what became known as bosons and went on to work with Albert Einstein to define one of two basic classes of subatomic particles. Much of the credit for discovering the boson, or "God particle," was given to British physicist Peter Higgs, much to the chagrin of the Indian government and people.
  • Determinated Woman: Lise Meitner

    Determinated Woman: Lise Meitner
    In 1938, Lise Meitner discovered that nuclear fission can produce enormous amounts of energy. She made the discovery in Sweden, after escaping a few months earlier from Nazi Germany. When World War 2 ended, she was acclaimed as the mother of the atom bomb. In fact, she disapproved of both the acclaim and the bomb.
  • Clever Woman: Rosalind Franklin

    Clever Woman: Rosalind Franklin
    Rosalind Franklin was a British biophysicist and X-ray crystallographer. Her discoveries related to the molecular structure of coal and carbon were used to develop strong carbon fibers and slow reactions in nuclear power plants.
  • Canadian: Richard F.W. Bader

    Canadian: Richard F.W. Bader
    Over his long career, he published 223 refereed articles and book chapters about chemistry and physics. Bader's works in recent years are cited more than 3000 times per year. Richard Bader discovered that electron density is very important in explaining the behavior of atoms in the molecules.
  • Canadian: Ronald J. Gillespie

    Canadian: Ronald J. Gillespie
    Ronald Gillespie is best known for his contribution along with Ronald Nyholm, in the development of Valence Shell Electron Pair Repulsion (VSEPR). The VSEPR is a set of rules that helps you to determine the geometric shape of a molecule, based on what molecule it is. He also helped to develop the LCP theory.
  • Canadian: Robert J. LeRoy

    Canadian: Robert J. LeRoy
    Le Roy is renowned for two major achievements in the field of chemistry: the development of the Leroy-Bernstein theory, alongside R. B. Bernstein, and the derivation of the LeRoy Radius. The LeRoy radius, derived by Robert J. LeRoy, defines the internuclear distance between two atoms at which LeRoy-Bernstein theory becomes valid.