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Democritus
In 450 B.C. Democritus’ atomic theory posited that all matter is made up of atomos, (a greek word meaning not divisible) small indestructible units. We now call them atoms. -
Law of Conservation of Mass
Law of Conservation of Mass is a relation stating that in a chemical reaction, the mass of the products equals the mass of the reactants. Basically, this law states that mass can neither be created or destroyed. This law was discovered by Antoine Lavoisier in 1785. -
Antoine Lavoisier
He is the father of modern chemistry. He proposed the Combustion Theory which was based on sound mass measurements. He named both oxygen and hydrogen and predicted silicon. Lavoisier's book, Traité Élémentaire de Chimie, (Elementary Treatise of Chemistry, 1789) was the first modern chemical textbook, it presented a unified view of new theories of chemistry, and contained a clear statement of the Law of Conservation of Mass. -
John Dalton
In the early 1800's, he determined that each chemical element is made up of a unique type of atom, and that the atoms differed by their masses. Dalton devised a system of chemical symbols and, having ascertained the relative weights of atoms, arranged them into a table. -
Dalton's Atomic Theory
Dalton's Atomic Theory has a few basic ideas:
All matter is composed of atoms and atoms can't be created or destroyed. All atoms of the same element are identical. Different elements have different types of atoms.Chemical reactions occur when atoms are rearranged.
Compounds are formed from atoms of the constituent elements. -
Dmitri Mendeleev
He created the first periodic table. The 60 elements were arranged according to their atomic weight. The elements were also arranged into what Mendeleev called groups. These groups had elements that had common characteristics. Mendeleev knew that not all elements were discovered yet, so in his periodic table Mendeleev left blank spots for the elements not yet found. -
Cathode Ray Tube
The cathode ray tube (CRT) is a vacuum tube containing an electron gun/emitter and a fluorescent screen used to view images. This helped J.J Thompson come up with his atomic theory. He did three experiments with the CRT. This also helped him discover the electron. -
Quantum Mechanical Model
The quantum mechanical model of the atom uses complex shapes of orbitals, volumes of space in which there is likely to be an electron. So, this model is based on probability rather than certainty. -
J.J. Thompson
J.J. Thomson determined the charge to mass ratio of electrons and he discovered the elctron. He also proposed the Plum Pudding Atomic Model. -
Plum Pudding Atomic Model
This model was proposed by J.J Thompson. It is also known as the Chocolate Chip Cookie or Blueberry Muffin Model. The model suggested a sphere of positive matter contains electrons which are positioned by electrostatic forces. -
Robert Millikan
He determined the unit charge of the electron with his oil drop experiment. e = 1.60 x 10-19 coulombs Millikan went on to demonstrate the photoelectron effect, which provided proof of Albert Einstein's equations. Millikan's experiments also aided both Einstein and Bohr in their later research efforts. -
Rutherford Model
This model was of course developed by Ernest Rutherford. The model described the atom as a "tiny, dense, positively charged core called a nucleus". In the nucleus nearly all the mass is concentrated,and around that are electrons. The electrons circulate at some distance. -
Gold Foil Experiment
Rutherford conducted this experiment. This experiment fired radioactive particles through thin metal foils (notably gold) and detecting them using zinc sulfide screens (a scintillator). Rutherford found that the majority of particles passed straight through the foil approximately, but 1 in 8000 were deflected. This lead him to his theory that most of the atom was made up of 'empty space'. -
Ernest Rutherford
He discovered alpha, beta, and gamma rays in radiation. In 1911 Rutherford's investigations, into the scattering of alpha rays and the nature of the inner structure of the atom, lead to his concept of the "nucleus". According to him the whole mass of the atom and all the positive charge of the atom is concentrated in a small space at the center. -
Bohr Planetary Model
The Bohr Model is a planetary model in where the electrons orbit a small positive charged nucleus. Its similar to the planets orbiting the Sun. The gravitational force of the solar system is mathematically like the Coulomb (electrical) force between the positivel charged nucleus and the electrons. This Model also says the inner layers can hold less electrons, than the outer layers. -
Electron Cloud Model
The Electron Cloud Model is an atom model where electrons are no longer thought of as moving around the nucleus in a fixed orbit. Instead, they are scattered and their exact location is unknown, and their location around the nucleus is described as an arbitrary ‘cloud’. -
Niels Bohr
In 1913 Bohr published the theory Bohr Planetary Model. This theory is about the structure of the atom based on an earlier theory of Rutherford's. Bohr expanded Rutherford's theory by proposing that electrons travel only in certain larger orbits. He suggested that the outer orbits could hold more electrons than the inner ones, and that the outer orbits determine the atom's chemical properties -
Henry Moseley
He studied x-rays given off of various elements. Each element produced different distinct set of wavelengths. Moseley showed that the x-ray wavelength could be better correlated with the atomic number. He then concluded that each element differs from the previous element by having one more postive charge in its nucleus. -
Erwin Schrodinger
In 1926, Erwin Schrödinger took the Bohr atom model one step further with the Quantum Mechanical Model and the Electron Cloud Model. -
James Chadwick
Chadwick repeated Frederic and Irene Joliot-Curie experiments but with the goal of looking for a neutral particle, one with the same mass as a proton (but with zero charge). Chadwick was successful and he determine that the neutron did exist and its mass was about 0.1% more than a proton's.
