Atomic Structure

  • Period: 400 to


  • 470

    Democritus, Greece.

    Democritus, a Greek philosopher, proposed the idea that there was a “void” in the world, or a “great nothing.” In his own words, he wrote, "by convention bitter, by convention sweet, but in reality atoms and void." He was the first to propose such an idea.
  • Dalton, London

    John Dalton formulated the Atomic Theory which states that: atoms of an element are different than atoms of other elements; atoms of one element are the same; that atoms of different elements can be combined to form more complex structures; also that atoms cannot be altered of destroyed; that atoms cannot be divided or separated; and that elements are made of tiny particles called atoms.
  • Mendeleev Periodic Table

    Dmitri Mendeleev arranged the elements with similar properties in seven groups. He also proposed the periodic law, stating that the properties of elements differ based on their atomic weights.
  • Henri Becquerel

    Henri Becquerel was the man who discovered radioactivity. He was conducting experiments on photographic film and discovered that some chemicals actually lose energy, or decompose, and then give off ionization particles and radiation. This decomposition produces a certain type of atom that radiates very penetrative rays.
  • Discovery of Electron

    Joseph John Thompson claimed that atoms could eject small negatively charged particles, which he named as electrons.
  • Atomic Nucleus

    Ernest Rutherford suggested that the nucleus of an atom was a small, highly condensed and positively charged structure. He was the first to assume the position of electrons around the nucleus.
  • Ernest Rutherford,

    Ernest Rutherford, in one of many experiments, used alpha particles as a kind of atomic bullet. He was able to prop the atoms present in a piece of gold tin foil. Doing this, he established that the nucleus was small, dense, and positively charged. He also found out that the electrons were located outside of the nucleus.
  • Charge in Nucleus

    H.G.J Moseley discovered the positive charge of the atomic nucleus. According to him, the atomic number of an element was equal to the number of protons.
  • Isotopes

    Aston discovered the existence of atoms with different number of neutrons for the first time by using mass spectrograph. He named them as isotopes.
  • Orbitals

    Niels Bohr described atomic structure and the presence of orbital shells. The atomic model consisted of successive shells in which the electrons were arranged.
  • Dual Nature of Atoms

    De Broglie discovered the dual nature of electrons. He proposed that the electrons could behave as waves or as particles.
  • Atomic Formula

    Heisenberg started the representation of atoms by formula. He founded the principle of indeterminacy stating that the position and velocity of a particle could not be determined at the same time.
  • Atom Model

    Schrodinger observed electrons as continuous clouds. He introduced wave mechanics in the atomic model.
  • Discovery of Neutrons

    James Chadwick discovered neutron by using alpha particles. He suggested that the mass of neutrons was similar to that of protons.