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442 BCE
Democritus
Democritus hypothesized that everything are composed of atoms, and that they are physically indivisible. Also that atoms are indestructible, are always in motion, and that there are an infinite number of atoms, differing in shape and size. -
John Dalton
He proposed the atomic theory.
The main points in Dalton's atomic theory are:
1. Atoms are small particles that make up elements
2. Size, mass and other properties are only identical in given atoms. Size, atoms and other properties differ in different elements.
3. Atoms are indivisible and indestructible.
4. Chemical compounds are formed from when atoms of different elements combine
5. Atoms are rearranged, combined or separated in chemical reactions. -
J.J. Thomson
Thomson made the discovery of what is known as electrons.
The exploration and the properties of cathode rays is how Thomson discovered subatomic particles.
He determined the charge to mass ratio of an electron
He developed the plum pudding model of the atom -
Max Planck
He contributed to the quantum theory. This theory helped the atomic and subatomic processes make more sense to humans.
Proposed that electromagnetic energy could only be released in quantized form. The theory formed the formula E=hv, which says that energy can only be a multiple of an elementary unit. -
Robert Andrews Millikan
He determined the charge and mass of an electron.
Altought the charge to mass ratio of the electron was discovered by J.J Thomson, Millikan's oil drop experiment had led to the discovery of the actual charge and mass values of an electron. -
Ernest Rutherford
He formulated the Rutherford model of the atom. In this model, there is a very small charged nucleus present. In this nucleus, there is much of the atom's mass. This nucleus is also being orbited by low mass electrons.
He made the Gold Foil Experiment to show a better structure of a atom. -
Niels Bohr
He made the Bohr atomic model, where it is similar to a planetary model. This model depicts the whole atom as a small, positively charged nucleus, that is present in the middle of the model. Around the nucleus, it is orbited by electrons that travel in a circular motion, similar to the solar system.
Bohr had proposed an atomic structure theory which stated that the outer orbit of an atom could hold more electrons than the inner orbit of the atom. -
Louis de Broglie
He made the matter and wave particle duality theory which set the basis of wave mechanics.
De Broglie had discovered that electrons had a dual nature. -
Werner Heisenberg
He created the quantom mechanics.
He proposed the uncertainly principle. This showed that knowing both the oposition and momentum of a particle at the same time is uncertain.
He also described atoms by means of formulas, that were connected to the frequencies of spectral lines. -
Erwin Schrödinger
He helped to form the basis of wave mechanics.
He formulated the wave equation that accurately calculated the energy levels of electrons in atoms.
Also he developed an electron cloud model. It consisted of a dense nucleus sorrounded by a cloud of electrons at various levels in orbitals.
