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430 BCE
Democritus discovered atoms
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. -
Dalton model/ Billard Ball model
Atoms cannot be broken down into anything simpler. All the atoms of a particular element are identical to each other and different from the atoms of other elements. Atoms are rearranged in a chemical reaction. Compounds are formed when two or more different kinds of atoms join together. He developed an atomic theory stating that all matter is made up of atoms. -
Period: to
Thomson model
Discovered the electron in 1897, proposed the plum pudding model of the atom in 1904 before the discovery of the atomic nucleus in order to include the electron in the atomic model. In Thomson's model, the atom is composed of electrons. -
Milikan Oil-drop experiment
He performed an "oil-drop" experiment, determined the charge of one electron and using the known mass-charge ratio, was able to calculate the mass on one electron. -
The Rutherford model
He preformed the gold foil experiment in which he demonstrated that the atom has a tiny, heavy nucleus. Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure. -
The Bohr model
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. -
Moseley's Law
He found that all atoms of a given element contained the same number of protons in the nucleus. -
Chadwick
James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron.
"Development of the Atomic Theory." Development of the Atomic Theory. N.p., n.d. Web. 02 Feb. 2017.
