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500 BCE
The Alchemists
The Alchemists developed the theory that all metals are composed of mercury and sulfur and that it is possible to change base metals into gold. -
400 BCE
Democritus
The theory of Democritus says that everything is composed of atoms, which physically, but not geometrically, cant be separated. that between atoms, there is only empty space, that atoms are indestructible, and have always been and always will be in motion, that there is an infinite number of atoms and of kinds of atoms. -
384 BCE
Aristotle
Aristotle did not believe in the atomic theory and he taught so otherwise. He thought that all materials on Earth were not made of atoms, but of the four elements, Earth, Fire, Water, and Air. He believed all substances were made of small amounts of these four elements of matter. -
347 BCE
Plato
Plato's cosmology derives from a mathematical discover by 'Empedocles'. Plato postulated that a fifth atomic type must exist which Aristotle later called `ether'. The stars, planets, Sun are composed of atoms of ether. -
330 BCE
solar system
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Robert Boyle
Boyle's Law states that if the volume of a gas is decreased, the pressure increases proportionally. Understanding that his results could be explained if all gases were made of tiny particles, he tried to construct a universal 'corpuscular theory' of chemistry. -
Lavoisier
Lavoisier found that mass is conserved in a chemical reaction. We now understand why matter is conserved atoms are neither created nor destroyed in a chemical reaction. -
John Dalotn
John Dalton 'hypothesized' that the law of conservation of mass and the law of definite proportions could be explained using the idea of atoms. He proposed that all matter is made of tiny particles that couldn't be split, called atoms, which he thought of as 'solid, massy, hard, impenetrable, movable particles' -
Dmitri Mendeleev
Mendeleev is known for his work on the Periodic Law and creation of the first Periodic table. He created the first Periodic Table. The Periodic Law says that when elements are arranged according to their atomic number, elements with similar properties will be shown at 'regular intervals'. -
"Plum Pudding" Model
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The Curies
In 1898 French physicists Pierre and Marie Curie discovered the strongly radioactive elements polonium and radium, which occur naturally in uranium minerals. Marie coined the term 'radioactivity' for the 'spontaneous emission' of ionizing, penetrating rays by certain atoms. -
J.J. Thomson
J.J. Thomson made the discovery of the electron. He is known for the Thomson atomic theory. Many scientists studied the 'electric discharge of a cathode ray tube'. It was Thomson's interpretation that was important. -
Albert Einstein
Albert Einstein mathematically proved the existence of atoms, and this helped change all the sciences through the use of statistics and probability. The Atomic theory says that any liquid is made up of molecules and these molecules are always in random motion -
Robert Millikan
His earliest major success was the accurate determination of the charge carried by an electron, using the “falling-drop method" he also proved that this quantity was a constant for all electrons, demonstrating the atomic structure of electricity. -
Solid Sphere of "Billiard Ball" Model
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Ernest Rutherford
Ernest Rutherford is known for his pioneering studies of radioactivity and the atom. He discovered that there are two types of radiation, alpha and beta particles, coming from uranium. He found that the atom consists mostly of empty space, with its mass concentrated in a central positively charged nucleus. -
Niels Bohr
Niels Bohr had 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 specific orbits. When jumping from one orbit to another with lower energy, 'a light quantum is emitted.' -
Henry G. J. Moseley
Henry Moseley was an English physicist who experimentally demonstrated that the major properties of an element are determined by the atomic number, not by the atomic weight, and firmly established the relationship between atomic number and the charge of the atomic nucleus. -
electron cloud model
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Werner Heisenberg
Werner Heisenberg contributed to atomic theory through 'formulating quantum mechanics' and in discovering the uncertainty principle, which states that a particle's position and momentum cannot be known exactly.
