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400 BCE
Democritus
Democritus discovered atoms, he said they were small particles that were all made of the same material. He concluded atoms are always in motion -
Antione Lavasier
Lavoisier developed an experimentally based theory of the chemical reactivity of oxygen and coauthored the modern system for naming chemical substances. -
John Dalton
John Dalton was a scientist that proposed the atomic theory in 1803. The atomic theory proposed that all matter was composed of atoms, indivisible and indestructible building blocks. While all atoms of an element were identical, different elements had atoms of differing size and mass. -
The solid sphere model
The Solid Sphere Model was the first atomic model and was developed by John Dalton in the early 19th century. He hypothesized that an atom is a solid sphere that could not be divided into smaller particles. He came up with his theory as a result of his research into gases. He realized that certain gases only combined in specific proportions. -
Dmitri Mendeleev
In 1907 Mendeleev arranged elements into 7 groups with similar properties. He discovered that the properties of elements "were periodic functions of their atomic weights". This became known as the Periodic Law. -
Sir J.J Thompson
Thompson identified the negatively charged electron in the cathode ray tube in 1897. He discovered that the electron was in all atoms. using a cathode ray. -
Robert Millikan
In 1909 Millikan determined the unit charge of the electron with his oil-drop experiment. Thus allowing for the calculation of the mass of the electron and the positively charged atoms. -
Sir Earnest Rutherford
In 1911 Rutherford studied alpha and beta particles, concluded they were concentrated in the nucleus. Using alpha particles and gold foil he created the nuclear model. He then discovered the nucleus was positively charged and discovered the neutron. -
Niels Bohr
Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well-defined quantities in 1913. -
Erwin Schrödinger
Schrödinger discovered the wave equation which used mathematical equations to describe the likelihood of finding an electron in a certain position. -
Sir James Chadwick
Using alpha particles discovered a neutral atomic particle with a mass close to a proton in 1932. Thus was discovered the neutron. In 1941, he wrote the final draft of the MAUD Report, which inspired the U.S. government to begin serious atomic bomb research efforts. -
Fredrich Hund
Hund determined the electron structure of atoms by developing a method that uses molecular orbitals in 1927. He then discovered and named Hund's rule, which is a rule based on observation of atomic spectra, which is used to predict the ground state of an atom or molecule with one or more open electronic shells. -
Wolfgang Pauli
In 1925 Pauli was able to prove this theory by introducing the concept of electron spin, he had discovered that each electron had a spin or intrinsic angular momentum. This spin of the electron allowed two electrons to occupy an orbital level in the atom. He made the Pauli exclusion principle, which states that in an atom no two electrons can occupy the same quantum state simultaneously. -
Nuclear model
Rutherford's atomic model became known as the nuclear model. In the nuclear atom, the protons and neutrons, which comprise nearly all of the mass of the atom, are located in the nucleus at the center of the atom. The electrons are distributed around the nucleus and occupy most of the volume of the atom -
Planetary model
Edwin Schrodinger created the planetary model in 1913, the model showed how negatively-charged electrons orbit a small, positively-charged nucleus similar to the planets orbiting the Sun. -
Electron cloud model
The electron cloud model was mad by Schrodinger in 1926 is a way to help visualize the most probable position of electrons in an atom. It is currently the accepted model of an atom.
