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465 BCE
Democritus
In the year of 465 B.C., Democritus hypothesized that atoms are indestructible, solid, but yet invisible, particles that construct everything in the universe. He also claimed that atoms different in shape, size, mass, and position and stated that small, pointy atoms constructed solids, large, round atoms made up liquids, and that oils were made of small, slippery atoms. Democritus is widely hailed as the father of the atom for this theory. -
John Dalton
John Dalton further added to the atomic theory by stating:
1. All elements are made of identical atoms
2. Compounds are made from two or more different elements
3. Compounds have fixed element ratios
4. All atoms have different weights and characteristics
John Dalton proposed most of the general information about atoms that we know and accept as fact nowadays. -
JJ Thomson
Thomson's discoveries began with his experimentations with cathode rays, beams that follow and electrical discharge in a high-vacuum tube. He was able to document the angle that the beams were deflected off at when the rays were passed through a vacuum and calculated the ratio of electric charge to particle mass. In this, he discovered that all gases are made up of similarly formed atoms, and eventually concluded that atoms were made up of small particles he called corpuscles. -
Ernest Rutherford
Ernest Rutherford is perhaps one of the more important scientists to the atomic theory. In his research he discovered that atoms emitted radioactive energy when both alpha and beta were emitted concurrently. Later, in 1907, Rutherford found out that all of an atom's mass is concentrated in its nucleus by performing his famous 'Gold Foil Experiment'. -
Rutherford Gold Foil Experiment
Ernest Rutherford's famous Gold-Foil Experiment was performed in 1907 when Rutherford took a sheet coated in a zinc-sulfide compound that would emit light once hit by a negatively charged particle. Rutherford then placed his assistant, Geiger, in a dark room before positioning a piece of 0.00004 cm thick gold foil in the middle of the ZnS sheet. He proceeded to bombard the gold foil with negative particles and, based on the flashes of light, Geiger found that the particles' scattering was small. -
Robert Millikan
Millikan is credited with the founding of the electron's charge. Millikan discovered that the electron was a negatively charged particle when he dropped drips of oil through two plates in an electric field, ionized the chamber by removing electrons from the air, and observing as oil drops caught the electrons and were attracted to the positive charged plate at the top of the chamber by either ascending, or descending slower. -
Neils Bohr
Bohr's contribution to the atomic theory was heavily debated and controversial, but eventually proved true and was the basis for the modern model of the atom. Bohr proposed that:
1. Electrons orbited the nucleus in set paths
2. The transfer of energy was defined indefinitely
3. Electrons made quantum jumps when exposed to energy that rereleased energy in the forms of light when the atoms dropped back to ground state. -
Erwin Schrödinger
While working with fellow scientist, Louis de Broglie, Schrödinger developed and discovered
1. His namesake wave equation that calculated the energy levels in electrons
2. That electrons both absorb and emit radiation at fixed wavelengths when jumping between orbits
3. Proposed that electrons stuck in orbits would emit standing waves (a vibration where some fixed points remain fixed and grounded but the others vibrate at their maximum amplitude. -
Werner Heisenberg
Werner Heisenberg's atomic model is still the accepted atomic model today. Heisenberg's work on the atomic model and theory includes:
1. That electrons absorb and emit radiation in fixed wavelengths while jumping between orbitals.
2. Formulated a type of Quantum Mechanics off the basis of matrices
3. Proposed the 'Uncertainty Principle' that one couldn't know the speed and location of a particle concurrently. -
James Chadwick
James Chadwick experimented for about a decade before finally contributing to the atomic theory. Chadwick is credited with the discovering the nucleus. He did this by bombarding Beryllium with alpha particles, and discovered that it emitted a stream of radiation. Originally thought to be gamma radiation, Chadwick disproved this and suggested that it might be Rutherford's neutron. This was confirmed later and he was granted a Nobel Peace Prize in 1935.
