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460 BCE
Democritus
He believed that atom types were determined by the shape of the atom. He thought that everything was made up of atoms and empty space. He was one of the first people to believe that atoms were everywhere. -
Period: 460 BCE to
Atomic Theory
The evolution of Atomic Theory spanning from 460BCE all the way to 1932. -
350 BCE
Alchemists
People who first practiced alchemy before chemistry. They came up with laboratory tools that are still used today such as funnels and flasks. They believed that atoms could not be destroyed and were always moving. -
John Dalton
One of the first chemists known for introducing the atomic theory, which was how all matter was composed of atoms. Born 1766 and died 1844. -
Antoine Lavoisier
He found the role that oxygen played in combustion and composed the phlogiston theory. He helped to make the metric system and helped to name some of its first units of measurement. 1743-1794 -
Law of Conservation of Mass
A law in atomic theory that states that in a chemical reaction, mass is neither created or destroyed. It was established in 1789. -
Law of Definite Proportions
A law that says that a given chemical compound always contains its component elements in fixed ratio and does not depend on its source. -
Joseph Louis Proust
French chemist who is best known for his law of definite proportions. He proved that relative quantities of any given compound’s constituent elements remain invariant. 1754-1826 -
Dalton's Atomic Theory
Dalton’s four theories that were based on the law of conservation of mass. The four theories were that all matter is made of atoms which are invisible, all atoms of a given element are identical in mass and properties, compounds are combinations of two or more different types of atoms, and that a chemical reaction is a rearrangement of atoms. -
Cathode Ray Tubes
Streams of electrons are observed in discharge tubes, the glass tubes are strapped with electrodes and the electricity is applied, the glass behind the positive electrode will glow, revealing a positive charge. This proved the existence of subatomic particles. -
Dmitri Mendeleev
He formulated periodic law and a version of the periodic table of elements. His table was the first that classified elements by their atomic number. 1834-1907 -
J.J Thomson
He was the first to think of the atom with positive and negative charges. He believed that if the atom had more positive or negative charges then the atom would be of that charge. 1856-1940 -
Plum Pudding Atomic Model
Model made by J.J Thomson, where the neutrons in his atomic model, look like plum seeds, surrounded by the pudding, which is the positively charged cloud, both of these within the atom itself. -
Gold Foil Experiment
Important series of experiments which gave scientists at the time insight to the nucleus within an atom. From this experiment they came to the conclusion that each atom has a nucleus where its mass and positive charge is held. They came to this conclusion by observing how an alpha particle beam was scattered when it struck a thin sheet of metal, which in fact was gold. -
Robert Milikan
Chemist who discovered the magnitude of the electron's charge. He found this using the oil drop experiment where he surveyed how drops of oil moved. 1868-1953 -
Oil Drop Experiment
This was the experiment that Robert Millikan used to find the magnitude of the electron's charge. The experiment involved a slightly charged drop of oil that was dropped between two metal plates and he would observe how the drops of oil moved. -
Nuclear Model
Rutherford’s atomic model has the protons and neutrons in the center with electrons orbiting around the protons and neutrons in an electron orbit. -
Ernest Rutherford
He created the nuclear model of an atom with the protons and neutrons in the middle while the electrons orbited around the protons and neutrons. He didn’t live long enough to fully prove his theory. 1871-1937 -
Niels Bohr
He proposed the theory that electrons orbit the nucleus on a predetermined route by using the models and theories at the time. 1885-1962 -
Planetary Model
Model of the atom created by Niels Bohr, where the neutrons orbit, as planets would, around a body in the center of the nucleus. This helps us to understand that neutrons can only hold and possess limited energy, and the neutrons constantly orbiting would be made possible by accelerated electrons. -
Henry Moseley
English physicist whose science contributions included discovering a systematic relation between wavelength and atomic number. 1887-1915 -
Quantum Mechanical Model
A proposal by Erwin Schrodinger that made a new model called the quantum mechanical model of an atom which treats electrons as matter waves. -
Erwin Schrodinger
Scientist who worked with Heisenberg to solve math equations that were used to prove Bohr’s theory of the atom by finding where the electrons were located. This led to them making the atomic model where the protons and neutrons are in the middle while the electrons are on the outside. This mapped out what the modern atom looks like. 1887-1961 -
Werner Heisenburg
A theoretical physicist who worked on quantum physics and developed key pieces of theoretical quantum notions. 1901-1976 -
James Chadwick
The first to discover neutrons using nuclear science. This lead to him helping with the making of the nuclear bomb by researching nuclear fission. 1891-1974
