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400
Democritus; Ancient Greece
"But in reality there are atoms and space." - Democritus introduces the philosophical concept of the atom in the universe. This begins the atomic theory - a theory of the nature of matter, which states that all matter is made up of atoms. -
Newton; England
Newton proposes that atoms are small solid substances constantly in motion in a 'mechanical universe' (an infinitely structured universe). This 'mechanical universe' is an important idea of Newton's, related to his explanation of the cosmic order. -
Dalton; England
Dalton proposes an 'atomic theory' with spherical solid atoms, based upon measurable properties of mass.
This 'atomic theory'of Dalton explores the features of the atom - its shape and state - as well as the idea of atomic mass. -
G.J Stoney; Ireland
Stoney proposes that electricity is composed of negative particles called electrons. This introduces the idea of electrons, linking it to electricity (hence establishing that electrons have a charge) and its structure. -
J.J Thomson; Britain
J.J Thomson's 'Plum Pudding' model (shown right), proposed before the discovery of the atomic nucleus.
In 1897, J.J Thomson uses a CRT (Cathode Ray Tube) to determine the ratio of mass to electrical charge of an electron. He studies canal rays (later to link them to the concept of isotopes).
Thomson's experiment provides more thorough understanding of the structure of an electron (its mass compared with its electrical charge) and its properties. -
Planck; Germany
Planck uses the idea of quanta (discrete units of energy - similar to atoms) to explain hot glowing matter. He then deduces the Quantum Theory, which explains the nature and behavior of nature and energy on an atomic and subatomic level. The Quantum Theory provides an insight on atoms in nature and energy. -
Nagaoka; Japan
Nagaoka's 'Saturnian' model of the atom (shown right).
Nagaoka creates a 'Saturnian' model of the atom with flat rings of electrons revolving around a positively charged particle. This model assists scientists in gaining a clearer mental image of the composition and structure of the atom and further develops their understanding of the electrons' positioning around the atom and atomic electrical charge.
This model is later proven to be incorrect, due to the inaccurate size of the nucleus . -
Millikan; U.S.A
Performs an 'oil drop experiment'. This experiment determines the exact charge and mass of an electron. The charge of the electron that Millikan experiments on proves that all electrons have negative charges. -
E. Rutherford; New Zealand
Performs an experiment: atoms in a very thin piece of gold foil are probed with alpha particles (they are split open).
This experiment establishes the nucleus as being very dense, very small and positively charged. From his observations Rutherford assumes that electrons are located outside the nucleus.
This experiment introduces the nucleus and its properties, and shed more light on the contents of the atom. -
H.G.J Moseley; England
By using X-ray tubes, Moseley determines the charge of the nuclei of most atoms. He discovers that "the atomic number of an element is equal to the number of protons in the nucleus".
This experiment helps him to create a more accurate version of the Periodic Table, reorganising it to base elements on atomic number instead of mass. -
Bohr; Denmark
He develops an explanation of atomic structure that underlies the regularities of the Periodic Table, bringing to recognition the trends and similarities that some elements share. Bohr creates a model of the atom, built up of successive orbital shells of electrons. -
James Chadwick; England
Using alpha particles in his experiment, Chadwick discovers the neutron in the nucleus of the atom. He notes that the neutron that he discovers has a mass close to the proton of the element's atom.
This discovery of Chadwick's introduces the neutron to the scientific community, contributing more knowledge about the atom's structure and composition.
