Atomic Timeline

Timeline created by radiniw
  • Period:
    -460 BCE
    -370 BCE


    Democritus hypothesized that all matter is made up of tiny, indestructible units, called atoms. The atoms in his theory remained unchanged, but move around in space to combine different ways to form all macroscopic objects. It stated that the characteristics of an object are determined by the shape of its atoms.
  • Period: to

    John Dalton

    Dalton suggested that atoms were like tiny, hard balls. Each chemical element had its own atoms that were different from others in mass. Dalton believed that atoms were the fundamental building blocks of nature and could not be split. Later he determined that compounds are made of molecules, and that molecules are composed of atoms in definite proportions. So, atoms determine the composition of matter, and compounds can be broken down into their individual elements.
  • Period: to

    Ernest Rutherford

    Rutherford's students, Hans Geiger and Ernest Marsden, did an experiment where the used dense, positively charged particles as 'bullets' to fire at a very thin piece of gold foil. Instead of going straight through, some rebounded off. Rutherford knew this could only mean one thing: The foil had a massive concentration of positive charge. He hypothesized that positive charge must be concentrated in a tiny volume at the center of the atom. On this model the electrons orbited around the nucleus.
  • Period: to

    J.J. Thomson

    J.J Thomson discovered the electron. He did experiments on the beams of particles in his tube. They attracted to a positive charge so he concluded that they must be negatively charged themselves. He said that the electrons must be embedded in a cloud of positive charge. THomson imagined the electrons as bits of plum in a plum pudding.
  • Period: to

    Niels Bohr

    Bohr knew there was no way to save an electron inside an atom from plummeting into the nucleus if it could give off energy continuously. He said that electrons can't radiate energy continuously. They only radiate in lumps. Bohr's model suggests that the electrons must be orbiting the nucleus in certain fixed energy levels. The energy must be given out when 'excited' electrons fall from a high energy level to a low one.
  • Period: to

    Werner Heisenberg & Erwin Schrodinger

    Heisenberg pointed out that is impossible to know the exact position and exact momentum of an object at the same time. This destroys the idea of electrons traveling around in orbits. Experiments conducted from the 1920 - 1950 lead to the creation of the charge-cloud model. The charge-cloud doesn't attempt to describe the path of each electron in a fixed orbit. Electrons are shown as 3D shapes that their probability fields define. The total paths of electrons are described as the electron cloud.