History Of Atomic Structure

  • 460

    Democritus said atoms cannot be destroyed and they exist in a vacuum or void.

    Atoms are indestructible, in between them lies empty space and they are constantly moving.
  • Newton proposed a mechanical universe with small solid masses in motion.

    Atoms are held together by attractions that we call forces.
  • Dalton Discovered atomic theory, he stated that “elements consist of tiny particles called atoms”.

    He said that atoms of the same element have the same mass, atoms of each element are different from one another and have different masses and he said compounds consist of atoms of different elements combined together.
  • G.J Stoney Proposed the term electron to describe the fundamental unit of electrical charge. And said that electricity was made of negative particles.

    His research laid foundations for the eventual discovery of the particle.
  • Planck Developed the quantum theory

    Electrons have fixed energy quantities.
  • Nagaoka Postulated a “saturnian” model of the atom with flat rings of electrons revolving around a positively charges particle.

    Electrons revolve around the positively charged nucleus.
  • Millikan Determined the charge and mass of an electron by using the oil-drop experiment.

    Electrons are negatively charged.
  • E.Rutherford Described the atom as having a central positive nucleus surrounded by negative orbiting electrons. He developed the coherent theory (Rutherford’s atomic theory).

    Atoms are comprised of a very small nucleus surrounded by electrons. Alpha radiation (positive charge), beta radiation (negative charge) and gamma rays (no charge)
  • Moseley argued that the frequencies of the x-rays should depend on the charge on the nucleus emitting these x-rays. Therefore, the atomic number was equal to the positive charge (charge on the nucleus) of an atom.

    proposed the name "proton" for the positively charged particles in the nucleus of an atom. At the same time, he also proposed that the nucleus also contained electrically neutral particles which accounted for the remaining mass of the atom. He called this yet unknown particle the "neutron".
  • Bohr said Electrons can only occupy certain orbits or shells in an atom. Each orbit represents a definite energy for the electrons in it.

    Modern concept of the atomic model.
  • James Chadwick Proved that neutrons, neutral particles in the nucleus that made up approximatelyhalf the mass of an atom, did exist.

    The neutron has no charge.
  • J.J Thomson Discovered the electron and suggested these particles were a building block of an atom.

    An atom contains electrons and that atoms are not invisible.