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460 BCE
Democritus
Democritus grew up in the city of Abdera in Thrace. He theorized that atoms were specific to the material that they are composed of, they were different shapes and sizes and could rebound or stick together. He knew that if a stone was cut in half, the two halves would have the same properties as the other half. Then, he reasoned that if the stone were to be cut into smaller pieces there would be a piece which would be so small it would be indivisible. -
John Dalton
John Dalton, born in 1776, was an English meteorologist and chemist who published his atomic theory in 1808. His theory had six main points. One, all matter consists of atoms. Two, different elements differ in size and mass. Three, atoms cannot be created or destroyed. Four, atoms of differing elements may combine in certain ratios to form compounds. Five, atoms of the same element can form two or more elements. Lastly, an atom is the smallest unit of matter that partakes in a chemical reaction. -
The First Evidence-Based Atomic Theory
John Dalton based his theory on laws of conservation of mass and the law of constant composition. His first theory stated that all matter is made up of atoms. His second theory stated that all atoms of a given element are identical in mass and properties. His third theory sated that compounds are combinations of two or more types of atoms. His fourth theory stated that a chemical reaction is a rearrangement of atoms. Parts of his theories had to be modified based on subatomic particles. -
J.J. Thomson
J.J. Thomson was an English physicist and Nobel Laureate in Physics born in 1856 in England. He's credited with the discovery and identification of the electron, the first subatomic particle discovered. He experimented with a Crookes tube to show that cathode rays were negatively charged and studied neon gas and the positive particles within it. Thomson realized that negatively and positively charged atoms were not accounted for in the current accepted model, resulting in his plum pudding model. -
Robert Millikan
An American physicist born in Morrison, Illinois. He went to college in Oberlin, Ohio and graduated, then went on to Columbia University to obtain his doctorate. He is most known for his studies of elementary electronic charge and photoelectric effect. Millikan began a series of experiments in 1909 to determine the electric charge carried by a single electron. His famous oil-drop experiment resulted in his most precise results. His experimental value is within 1% of the current accepted value. -
Ernest Rutherford
Ernest Rutherford lived from August 30, 1871, to October 19, 1937, and lived in New Zealand. He was experimenting with uranium and he placed it near foil and discovered two radiations call "alpha" and "beta" and found out that the alpha was identical to the nucleus of a helium atom, and the beta particle was the same as an electron or a positron. He was awarded, Rumford medal, Copley medal, Bressa prize, Albert medal, and the faraday medal. -
Neils Bohr
Lived in Copenhagen, Denmark until November 9, 1962. He discovered that based on the quantum theory energy is transferred only in certain quantities. Electrons move around the nucleus but only in specific orbits. When moving from one orbit to another with lower energy, a light quantum is produced. Bohr was the 1st to discover that electrons travel in separate orbits around the nucleus and that the amount of electrons in the outer orbit determines the properties of an element. -
Erwin Schrodinger
Erwin Schrodinger was an Austrian physicist and Nobel Laureate born on August 12, 1887. He developed many fundamental results of quantum theory. One being the Schrodinger Equation, which provides a way to calculate the allowed energy levels of quantum mechanical systems. He was also an author, whose works covered many various fields of physics. Additionally, he focused on philosophical aspects of science, one of his most famous experiments being the "Schrodinger's cat" thought-experiment. -
James Chadwick
James Chadwick was born on October 20, 1891, and died on July 24, 1974. He lived in England and also went to school there. In 1932, he discovered and proved the existence of neutrons. Like the helium nuclei (alpha rays), which are charged, therefore they move away by the electrical forces present in the nuclei of heavy atoms. Doing this he prepared a way towards the fission of uranium 235 and towards the creation of an atomic bomb. -
Electron
The electron was discovered by J. J. Thomson in 1897 when he was studying the properties of the cathode ray. -
Cathode Ray tube experiment
J. J. Thomson discovered the existence of electrons. He did this using a cathode ray tube, which is a vacuum-sealed tube with a cathode and anode on one end that create a beam of electrons travelling towards the other end of the tube. -
Size and Charge of an Electron
It was discovered by a British physicist named Sir Joseph John Thomson. Later, in 1913, an American Physicist by the name of Robert A. Millikan created an accurate measurement of the electron's charge. Recent studies show that the charge of an electron is 1.60218 × 10−19 coulombs. -
Gold Foil Method
The Rutherford Gold Foil experiment shot particles at a thin sheet of gold. It was found that a small percentage of the particles were deflected, while a majority passed through the sheet. This caused Rutherford to conclude that the mass of an atom was concentrated at its center. -
Oil Drop Experiment
Performed by Robert A. Millikan and Harvey Fletcher in 1909 to measure the elementary charge of the electron. The experiment was when he observed tiny electrically charged droplets of oil between two parallel metal surfaces, forming the plates of a capacitor. -
Neucleus
The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. -
The Neutron
The neutron was discovered by James Chadwick in 1932 and the idea that it was a new elementary particle, distinct from the proton.
