December 10, 2020
For educational institutions
Atomic Theory Timeline Project
⟶ Updated 21 Oct 2018 ⟶
List of edits
Antoine Lavoisier: Discovered the law of conservation of mass which states that mass can neither be created nor destroyed in a chemical reaction; Mass is conserved in a chemical reaction; The total mass of the products of a chemical reaction is always the same as the total mass of the starting materials consumed in the reaction
Plum Pudding Model: Created by J.J Thomson; Negative electrons represent raisins, embedded in the dough (a positively charged substrate); The atom is composed of electrons surrounded by a soup of positive charge to balance the electrons' negative charges
Bohr Planetary Model: Created by Niels Bohr; Depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus; Electrons orbit the nucleus in orbits that have a set size and energy; The energy of the orbit is related to its size. The lowest energy is found in the smallest orbit; Radiation is absorbed or emitted when an electron moves from one orbit to another
Democritus created the Atomic Theory: Everything is composed of “atoms”, which are physically, but not geometrically, indivisible; Between atoms, there lies empty space; Atoms are indestructible; Atoms have always been, and always will be, in motion; There are an infinite number of atoms, and kinds of atoms, which differ in shape, and size.
J.J. Thomson: Discovered the electron by discovering that cathode rays are negatively charged; Created the plum pudding model
John Dalton discovered All matter is made of atoms; Atoms are indivisible and indestructible; All atoms of a given element are identical in mass and properties; Compounds are formed by a combination of two or more different kinds of atoms; A chemical reaction is a rearrangement of atoms
Gold Foil Experiment: Aimed alpha particles at gold foil and recorded their location as they passed through the foil-> a small number of particles ricocheted back; Rutherford concluded the atom consisted of a small, dense, positively charged nucleus in the center of the atom with negatively charged electrons surrounding it.
Cathode Ray Tube Experiment: Done by J.J Thomson; Found that many different metals release cathode rays, and that the rays were made of electrons, very small negatively charged particles; He constructed a cathode ray tube, with a fluorescent coating at one end and a vacuum. Halfway down the tube were two electric plates, producing a positive and negative cathode. The rays were deflected by the electric charge, proving that the rays were made up of charged particles carrying a negative charge
Robert Millikan: Performed the oil drop experiment; Measured the charge of an electron; Offered convincing proof that electric charge exists in basic natural units
James Chadwick: Played a vital role in the atomic theory, as he discovered the Neutron in atom; Neutrons are located in the center of an atom, in the nucleus along with the protons. They have neither a positive nor negative charge, but contribute the the atomic weight with the same effect as a proton.
Erwin Schrodinger : Invented the quantum mechanical model of the atom, which describes the likelihood of finding an electron in a certain position.
Niels Bohr: Proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities; Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted.
Oil Drop Experiment: Performed by Robert A. Millikan and Harvey Fletcher; Observing tiny electrically charged droplets of oil located between two parallel metal surfaces, forming the plates of a capacitor to measure the electric charge of an electron; Offered convincing proof that electric charge exists in basic natural units
Law of Conservation of mass: Created by Antoine Lavoisier; States that mass can neither be created nor destroyed in a chemical reaction.
Dalton's atomic theory: Created by John Dalton; Proposed the theory that all matter was composed of atoms, indivisible and indestructible building blocks; While all atoms of an element were identical, different elements had atoms of differing size and mass.
Rutherford Model: Created by Ernest Rutherford; Proved J.J Thomson’s plum pudding model incorrect; Proposed that an atom is composed of empty space mostly with electrons orbiting in a set, predictable paths around fixed, positively charged nucleus
Electron Cloud Model: The electron cloud model was developed by Erwin Schrödinger; It consisted of a dense nucleus surrounded by a cloud of electrons at various levels in orbitals; Where the cloud is most dense, the probability of finding the electron is greatest, and the electron is less likely to be in a less dense area of the cloud
Ernest Rutherford: Was the first to discover that atoms have a small charged nucleus surrounded by largely empty space, and are circled by tiny electrons, which became known as the Rutherford model (or planetary model) of the atom.
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