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800 BCE
Start of Timeline
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450 BCE
Aristotle
He thought that all materials on Earth were not made of atoms, but of the four elements, Earth, Fire, Water, and Air. -
400 BCE
Democritus
He theorized that all material bodies are made up of indivisibly small “atoms.” -
400 BCE
Leucippus
Earliest Greek to develop the theory of atomism -
Robert Boyle
Discovered Boyle's law expresses the inverse relationship that exists between the pressure and volume of a gas -
Antoine Lavoisier
Lavoisier found that 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. -
Antoine Lavoisier
Lavoisier reported that water was roughly 85% oxygen and 15% hydrogen by weight. Water therefore seemed to contain 5.6 times more oxygen by weight than hydrogen. -
Joseph Proust
Discovered of the law of definite proportions in 1793 -
John Dalton
1.) The first part of his theory states that all matter is made of atoms, which are indivisible and indestructible
2.) The second part of the theory says all atoms of a given element are identical in mass and properties.
3.) The third part says compounds are combinations of two or more different types of atoms.
4.) The fourth part of the theory states that a chemical reaction is a rearrangement of atoms. -
John Dalton
Discovered the law of multiple proportions, also known as Dalton's law, -
Amadeo Avogadro
1st fact - In 1811 he claimed that equal volumes of all gases at the same temperature and pressure contain the same number of molecules. This is now called Avogadro’s law.
2nd fact - His experiments in 1811 helped inspire Avogadro's constant which he did not create -
Michael Faraday
Made one of the most significant discoveries that led to the idea that atoms had an electrical component. Faraday placed two opposite electrodes in a solution of water containing a dissolved compound. -
Dmitri Mendeleev
1st fact - In 1869 he created the first Periodic Table.
2nd fact - In 1869 he discovered periodic law also called Mendeleev's law -
George Stoney
He had introduced the concept of the electron, though not the word, as early as 1874, initially naming it "electrine" -
Eugen Goldstein
Discovered positive particles or rays in 1886 -
George Stoney
Introduced the term electron as the "fundamental unit quantity of electricity" in 1891 -
Jean Baptiste Perrin
In 1895 he settled the cathode ray particle-wave debate by demonstrating that cathode rays carry negative charges and are thus particles. -
Henri Becquerel
Discovered the radioactivity in uranium in 1896 -
Marie Curie
Marie came up with a new term called “radioactive.” in 1896 -
J.J. Thomson
J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. -
Pierre Curie
1st fact - In 1898 along with Marie discovered polonium
2nd fact - In 1898 along with Marie discovered radium -
Marie Curie
1st fact - In 1898, Marie discovered a new element that was 400 times more radioactive than any other. They named it “polonium,”
2nd fact - In 1898 announced the existence of another element they called “radium,” It gave off 900 times more radiation than polonium. -
Max Planck
Discovered the quantum of action, now known as Planck's constant, h, in 1900 -
J.J. Thomson
Thomson proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged "soup." -
Otto Hahn
1905 Hahn discovered a new substance he called radiothorium (thorium-228), which at that time was believed to be a new radioactive element. -
Hans Geiger
In 1908, Geiger introduced the first successful detector of individual alpha particles. -
Robert Millikan
Millikan began a series of experiments to determine the electric charge carried by a single electron. The results suggested that the charge on the droplets is a multiple of the elementary electric charge -
Ernest Rutherford
In 1909 Rutherford disproved Sir J.J. Thomson's model of the atom as a uniformly distributed substance. -
Otto Hahn
In 1909, Hahn discovered radioactive recoil. -
Ernest Rutherford
1st fact - made the Rutherford model which is a description of the structure of atoms proposed in 1911
2nd fact - discovered protons in 1911
3rd fact - discovered the nucleus in 1911 -
Niels Bohr
1st fact - In 1913, Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities.
2nd fact - 1913 made the Bohr Atomic Model which looks like a solar system -
Henry Mosely
1st fact - In 1913 he used self-built equipment to prove that every element's identity is uniquely determined by the number of protons it has.
2nd fact - In 1913 he predicted confidently the existence of four new chemical elements, all of which were found. -
Frederick Soddy
Conclude in 1913 that certain elements might exist in forms that differ in atomic weight while being indistinguishable and inseparable chemically. Called isotopes -
Louis de Broglie
In 1924 Louis de Broglie introduced the idea that particles could be described not only as particles but also as waves. -
Erwin Schrӧdinger
1st fact - In 1926 Erwin Schrödinger formulated a wave equation that accurately calculated the energy levels of electrons in atoms.
2nd fact - 1926 Erwin Schrödinger proposed the electron cloud model of the atom -
James Chadwick
In 1932 Chadwick discovered neutrons -
Otto Hahn
He inadvertently discovered nuclear fission during an experiment in which the uranium atom split into barium -
Lise Meitner
In 1938, Lise Meitner discovered that nuclear fission can produce enormous amounts of energy -
Lise Meitner
In 1939 coined the term nuclear fission -
End of Timeline