History of the Atom


Dalton finalized atomic theory research

  • Research suggested that particles making up different gasses must have different masses
  • Research led to Atomic Theory Atomic Theory: first complete attempt to describe matter in terms of atoms and properties

Dalton publishes his model of an atom


-Known as “Billiard Ball Model”
-Model was inaccurate because it the atom as simple sphere without protons, electron or neutrons.

JJ Thompson discovers Cathode-Ray particles are negatively charged


JJ Thompson discovers Cathode-Ray particles are at least 1000 times lighter than one hydrogen atom


JJ Thompson discovers all ray particles have an identical mass and charge


Rutherford publishes Theory Of Atomic Disintegration


-Rutherford discovered that radioactive emission involved spontaneous disintegration of atoms into other types of atoms.

JJ Thompson’s model of an atom is published

  • Known as “Plum Pudding Model”
  • Included the idea that negatively charged electrons are surrounded by positively charged areas

JJ Thompson discovers that every hydrogen atom has one electron


-Electrons are carrier of universal unit of charge

Milikan performs Oil Drop Experiments

  • Milikan measured the amount of electric force and magnitude of the electrons field on charge of isolated droplet
  • Milikan is able to prove the size of the charge itself

Rutherford’s model of an atom is published


Rutherford performs experiment which leads to discovery of the proton


-Experiment suggest the nucleus of an atom contains a particle with a positive charge

Millikan verifies Einstein’s photoelectric equation and makes the first direct photoelectric determination of Planck’s constant

1912 - 1915
  • Photoelectric effect: when light falls onto metal surface and ejects electrons

JJ Thompson proves that stable elements can exist as isotopes


Milikan determined that maximum kinetic energy of ejected electrons follow kinetic energy of particles equation


-½ mv (squared) =hf-P

Bohr suggests that electrons orbit atom’s nucleus in shells with a fixed radius


Bohr’s model of an atom is published


De’ Broglie proposes the Wave Particle Duality Theory of Matter

  • particles could be treated as matter waves with wavelengths shown through formula λ = h/mv.
  • applied theory to explain a previous model of the atom called the Bohr model

Schrodinger creates the quantum mechanical (AKA wave mechanical) model

  • Electrons move in orbitals
  • H​​ψ=Eψ is used to calculate probability of finding an electron in a certain area

Bohr explains nuclear fission with Liquid Droplet Theory

  • Liquid Droplet Theory: A liquid droplet of an element is able to provide an accurate model for an atom’s nucleus. The protons and neutrons of an atom behave like the molecules of the atoms behave inside a drop of liquid.