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460
Origin of the Atomic Theory - 460 B.C.
The Atomic theory is believed to have been discovered in Greece by Leucippus of Miletus and his student Democritus of Abdera. The word atom comes from the greek word atomos meaning “invisible.” These greeks held that, in addition to being small, unchangeable, and indestructable, atoms were also completely solid, with no internal structure and came in an infinite variety of shapes and sizes. -
Period: 460 to
Democritus to John Dalton
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John Dalton
John Dalton came up with the Solid Sphere model, which showed all atoms are seen as solid.
1. Elements are composed of atoms
2. All atoms of an element are identical and have the same mass
3. Atoms of different elements have different weights
4. Elements combine to form compounds in fixed ratios
5. Atoms cannot be created, divided, or destroyed (conservation of mass)- chemical reactions only rearrange them -
J.J. Thomson
J.J. Thomson contributed the existence of electrons. That meant that atoms are not invisible and could be seen. His theory also suggested that atoms where spherical and had a positive charge. Also revealed his Plum Pudding model. -
Albert Einstein
Einstein contributed to the atomic theory in a very important way. Instead of just theorizing that atoms may possibly exist, he actually found a way to prove that they did. He did this by using his theories in quantum physics to explain the photoelectric effect and Brownian motion, which ultimately led to solid evidence that atoms exist and even showed how atoms are structured. -
Robert Millikan
Robert Millikan, and a former student of his, did and oil drop experiment to calculate the charge of an electron. -
Ernest Rutherford
Rutherford's experiment showed that something dense and positively charged existed in the atom. We call this the nucleus. He also proved that most of the atom was made of empty space. Rutherford also revealed his Nuclear Model. -
Neils Bohr
Neils Bohr introduced his Planetary Model in 1913. Which depicts the nucleus with electrons surrounding it. -
Louis de Broglie
In 1924 Louis de Broglie suggested that all matter has wave properties. -
Wolfgang Pauli
Pauli Exclusion Principle - no two electrons in an atom can be at the same time in the same state or configuration. -
Erwin Shrodinger
In 1926 Erwin Shrodinger took the Bohr model 1 step further by using mathematical equations to decribe the liklihood of finding an electron in a certain position. -
Werner Heisenberg
In 1927 Werner Heisenberg introduced the Uncertainty Principle which states the exact location and energy of an electron cannot be determined simultaneously. Finding one value (the energy or the location), means that there will be some uncertainty when finding the other value. -
James Chadwick
In 1932 James Chadwick discovered the Nuetron. A neutron is a particle with no charge.
