Atomic mass 3024

Atomic Theory Time Line

  • 340


    Aristotle did not believe in the atomic theory. He thought that all matter on earth were not made of atoms, but of the four elements, Earth, Fire, Water, Air, and Aether He believed every single thing on earth was made of from small bits of these elements. It was because of Aristole's teachings that Democritus' theroy was overlooked for nearly 2,000 years.
  • 400

    Invisible solid sphere atomic model

    Invisible solid sphere atomic model
    Democritus' model of the atom. Most Greeks believed atoms were solid invisible spheres.
  • 465


    Democritus theorized 3 laws of atoms:
    • Atoms are indestructible
    • All matter is made up of atoms
    • Atoms are invisible solids
  • Lavoisier

    Created the law of the conservation of matter.
    Law of the conservation of matter: Matter may change form, but cannot be created nor destroyed.
    Named oxygen
  • Proust

    Created the Law of Definite Proportions.
    Law of Definite Proportions: Compounds are composed of exact rations of elements by mass.
  • Faraday

    Studied the effect of electricity on solutions, invented the term "electrolysis" as a splitting of molecules with electricity, developed laws of electrolysis although he did no advocate atomism
  • Dalton

    Theorized that all elements were different because they had different masses. Atoms were miniscule, indivisible particles that could not be destroyed and differed in size, chemical behaviour and mass depending on its element.
  • Becquerel

    While studying the effect of x-rays on photographic film, he discovered some chemicals spontaneously decompose and give off very pentrating rays. MArie and Pierre Curie later further experimented his findings.
  • Thomson

    Discovered the electron and proved his theory using the cathode ray tube. This showed that atoms were made of two subparticles. The proton and the electron, proving that atoms were actually divisible.
  • Curie

    Discovered Po and Ra in 1898. Studied the radioactivity in elements. During WWI Marie Curie dedicated herself to working on the development of X-rays.
  • Plum pudding model

    Plum pudding model
    Model by J J Thomphson. Believed the atom was positive with negative electrons inside.
  • Milikan

    Discovered the charge of a single electron.
    Used the oil drop experiment to calculate this.
  • Rutherford

    Discovered alpha and beta rays. He proposed the nuclear structure of the atom:
  • Einstein

    Expressed the equivalency of mass and anergy in an equation.
    Accurately calculated the average distance an immersed visible particle would travel in a given time.
  • Planetary model

    Planetary model
    Model by Rutherford. Believed an atom was a "central charge" with an electron cloud around it.
  • Planck

    Used the idea of quanta to explain hot glowing matter.
    E = hν
    Planck's constant is the energy E is equa to the frequency v, and the proportionality constant h,
  • Electron cloud model

    Electron cloud model
    Model includes a cloud-like region where the electrons should be, but does not clearly show them. Based on probability of an electron's location. Created by Schrödinger.
  • Schrödinger

    Assumes electrons are waves. Created an atomic model that tells us where electrons can be, not their location. Formed a wave equation (Schrödinger's equation) that can accurately give an atom's energy level.
  • Quantum mechanics model

    Quantum mechanics model
    Showed the mathematics of the quantum theory applied to an atom in the form of a model. Created by Schrödinger.
  • De Broglie

    De Broglie
    Greatly improved the knowledge of the physical nature on the atomic scale. He believed that electrons can act like both particles and waves, just like light. Electrons create waves when in orbit around a nucleus.
  • Chadwick

    Discovered the existence of neutrons.
  • Bohr

    Discovered that electrons travel in separate orbits around the nucleus and the number of electrons in the outer orbit determines the properties of an element. Liquid droplet theory: a liquid drop provides an accurate representation of an atom's nucleus. Quantum theory: an electron can be viewed in two ways, either as a particle or as a wave, but never both at the same time.
  • Hahn

    Discovered radioactinium. Discovered that the element barium was produced when uranium atoms were bombarded with neutrons. This discovery indirectly helped develop the atomic bomb and nuclear energy.
  • Meitner

    Conducted experiments verifying that heavy elements capture neutrons and form unstable products which undergo fission. This process ejects more neutrons continuing the fission chain reaction.
  • Seaborg

    Produced six transuranium elements and suggested a change in the layout of the periodic table. Founded neptunium and predicted the exsistence of plutonium. Worked on the Mathattan project.