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475 BCE
Democritus
Democritus was born around 475 B.C.E. and was born in either Abdera or Miletus. He was one of the first known people to believe that the solidity of a material corresponded with the shape of the atoms involved. He also popularized and illustrated the concept of atomic theory. Democritus died around 370 B.C.E. -
700
Alchemists
Alchemists tried to transform base metals into more valuable metals, specifically silver and gold, and to discover a cure for disease and a way of extending life. Alchemy originated in Ancient Egypt and spread to the western world in the 8th century. It was also developed in China by Taoist monks. By the 16th century, alchemists had separated into two groups. One group was the group that evolved into chemists. Alchemy ended in the 18th century and was replaced by modern chemistry. -
Antoine Lavoisier
Born on August 26, 1743, Lavoisier was a very significant French chemist. He developed the Law of Conservation of Mass and discovered that combustion was created from a reaction with a metal or organic substance and part of the air. He called this part of the air oxygen. He disproved the popular theory of phlogiston. Lavoisier died on May 8, 1794. -
Joseph Louis Proust
Joseph Louis Proust was born in 1754. He developed the Law of Definite Proportions, which states that quantities of given pure chemical compounds do not change. He performed experiments with inorganic binary compounds, like metallic oxides, sulfides, and sulfates. He died on July 5, 1826. -
John Dalton
He studied meteorological properties and determined that the atmosphere was 80% nitrogen and 20% oxygen. He also formulated the Law of Multiple Proportions and claimed that the atoms of different elements were different sizes. He died on July 27, 1844. -
Law of Conservation of Mass
This law states that matter cannot be created or destroyed in a chemical reaction. The reactants should have the same mass as the product. -
Law of Definite Proportions
Developed by Joseph Proust, this law states that every chemical compound contains fixed and constant proportions of its constituent elements. -
Dalton's Atomic Theory
This theory proposed that all matter was composed of atoms, which were indivisible and indestructible. All atoms are identical, but they have a different mass and size. -
Dmitri Mendeleev
Dmitri Mendeleev was a Russian chemist who developed a periodic classification of elements. He made a version of the periodic table in 1871 and left blank spaces for elements that had not been discovered yet. He discovered gallium, scandium, and germanium. -
J.J. Thomson
Born in 1856, Thomson was a British physicist who investigated cathode rays. He proposed that the rays were made of particles that were smaller than atoms. He was correct. The rays were made of electrons. He discovered the electron in 1897. -
Robert Millikan
Millikan was well-known for his oil-drop experiment. He won a Nobel Prize in 1923 for his study of the elementary electronic charge and the photoelectric effect. He died in 1953 -
Ernest Rutherford
Rutherford is most well-known for his gold foil experiment and his model of the atom. He discovered that the atom is mostly empty space and that all of the positive material is gathered in the middle. -
Niels Bohr
Bohr applied quantum concepts to atomic structure issues and won a Nobel Prize for his atomic model. He died on November 18, 1962. -
Henry Moseley
Born in 1887, Mosley experimented with radioactivity. He also established Moseley's Law, which states that frequencies are proportional to the squares of the numbers that are equal to the atomic number plus a constant. He died on August 10, -
Erwin Schrödinger
He demonstrated that the quantization of a hydrogen atom's energy levels (as demonstrated in Bohr's model) can be calculated from the Schrödinger equation. The equation describes the wave of a quantum mechanical function. -
James Chadwick
James Chadwick won a Nobel Prize for discovering the neutron in 1932. He also wrote the final draft of the MAUD report, which inspired the U.S. government to begin serious atomic bomb research. He died in 1974. -
Cathode Ray Tube
This was invented by J.J. Thomson and was used to discover the electron. It consisted of two plates midway in the CRT connected to an electric battery, which created an electrical field through which the rays passed. -
Werner Heisenberg
He formulated quantum mechanics in terms of matrices and was awarded the Nobel Prize for Physics in 1932. He developed the uncertainty principle, which stated that nature is unclear and cannot be determined exactly. He died in 1961. -
Plum Pudding Atomic Model
This atomic model was created by J.J. Thomson in 1903. In this model, the "pudding" is the positive material. The "raisins" were the negative particles. -
Oil Drop Experiment
This was performed by Robert Millikan and Harvey Fletcher. It was used to measure the charge of an electron. It was originally used with water, but it was changed to oil after the water evaporated too quickly. -
Nuclear Model
In 1911, Rutherford developed this model, which demonstrated that there was a lot of empty space in the atom as well as a concentrated area of positive material. The electrons are distributed around the nucleus. -
Gold Foil Experiment
This experiment was conducted by Ernest Rutherford and his co-workers. It was intended to determine the angles through which a beam of alpha particles would scatter through a sheet of gold foil. He found that the vast majority of alpha particles behaved as expected, but some were reflected on angles over 90 degrees. -
Planetary Model
This model was created by Niels Bohr in 1913. There was a dot in the center (nucleus) and proved that there was mostly empty space in the atom. -
Quantum Mechanical Model
This model uses complex shapes of orbitals and volumes of space that are likely to contain electrons. Created by Erwin Schrödinger, this model is based a lot off of probability.
