Honors Chemistry Atomic Theory Timeline

Atomic Theory

Period: 460 BCE to Apr 22, 2016

Leucippus of Abdera

450 BCE

Very little is known about he fifth century BC born Greek philosopher. Though he was credited to be the first to theorize the existence of atoms. Though very little is known about him, it may have been possible he had relations with Democritus.

Democritus

400 BCE

Though Leucippus may have been the first person to believe in atomic theory, Democritus was still credited as the first coin the term "atomos" which is what the modern word for atom is derived from. He has stated that “The universe is composed of two elements: the atoms and the void in which they exist and move.”

Aristotle

340 BCE

Aristotle believed that there were 4 elements which were water, earth, fire, and air (5 elements actually if you count Aether which he considered to be the element that made up stars and celestial bodies) that made up everything, each which had different physical properties. Unlike Democritus, Aristotle believed that there was no singular smallest unit of matter and that the elements will continue to decrease in size the more they are cut.

Note:

100

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Alchemists

Apr 24, 600

These early "pseudo-chemists" theorized that all metals are made up kinds of sulfur and mercury and that could be changed into gold.

Isaac Newton

Jul 2, 1704

Though he did not focus his work on developing an atomic theory, Newton did propose that in this universe there are small particles of solid mass that were always constantly in motion in his book "Optics" in 1704. He contributed the idea that these tiny solid "massy" particles are held together with different tangible forces.

Benjamin Franklin

Jun 2, 1749

While he did not focus his work on the development of the atomic theory, his studies in electricity did help contribute to it. What Franklin discovered was through the rubbing of fur on glass and rubber rods he observed the two charges of electricity which later helped formulate the science behind protons and electrons.

Antoine Lavoisier

Jun 2, 1789

He conducted experiments turning HgO into Hg+O. Through this he developed the Law of Conservation. The law states that matter cannot be made or destroyed. He supported the rearrangement of matter in reactions.

Joseph Proust

Jun 2, 1799

Proust stated the Law of Definite Proportions. The law states that the ratio of elements in a compound will always be the same and constant. His proposal helped Dalton to create the Law of Multiple Proportions and ultimately his atomic theory.

John Dalton

Jun 2, 1803

Dalton proposed significant ideas in the atomic theory and was the first to do so in a long time. His rules are still applied today. They were:
1) All matter is made of atoms. Atoms are indivisible.
2) All atoms of an element are identical in mass and properties
3) Compounds are formed by a combination of two or more different kinds of atoms.
4) A chemical reaction is a rearrangement of atoms.
His model for the atom was still simplistic as subatomic particles had not been discovered yet.

Michael Faraday

Jun 2, 1832

Faraday studied the effect of electricity on solutions. He was the one who coined term "electrolysis" as a splitting of molecules with electricity. Through his discovery in the electromagnetic field we were able to perform more accurate studies of the atom,

Dmitri Mendeleev

Jun 2, 1869

Mendeleev published the periodic table of elements which included the all the known elements at the time. It was arranged into 7 groups with similar properties. He developed the Periodic Law.

James Clerk Maxwell

Jun 2, 1873

Maxwell published physical and mathematical theories of the electromagnetic field. He postulated that electric and magnetic fields filled the empty space within an atom.

Sir William Crookes

Jun 2, 1879

Crookes conducted experiments with cathode rays. He found that they travel in straight lines from the cathode but bend in path when different magnetic poles are applied to the tube. He concluded that the rays had a negative charge and that it had mass because it caused pinwheels to spin.

Eugen Goldstein

Jun 2, 1886

Goldstein noted that cathode-ray tubes have a glow at the end of the tube near the cathode. He concluded that in addition to the negative particles (electrons), that traveled from the cathode toward the anode, there is another ray that travels in the opposite direction. Goldstein called them canal rays. which must have positive charges.

Werner Heisenberg

Jun 2, 1891

He proposed Principle of Uncertainty- you can not know both the position and velocity of a particle. This contributed to both quantum mechanics and atomic theory.

Henri Becquerel

Jun 2, 1896

While studying the effect of x-rays on photographic film, Becquerel discovered some chemicals spontaneously decompose and give off very penetrating rays. This led to the discovery of radiation. This was significant to the atomic theory because it led to explaining how atoms decay.

Joseph John Thomson

Jun 2, 1897

Thomson discovered and named the electron as well as isotopes. He also invented the mass spectrometer. This helped develop atomic theory further. He also developed the plum pudding model which is now outdated.

Marie Sklodowska Curie & Pierre Curie

Jun 2, 1898

The couple studied uranium and thorium and observed spontaneous decay. They called the process "radioactivity". The two contributed to atomic theory by discovered the radioactive elements radium and polonium.

Hantaro Nagaoka

Jun 2, 1903

Nagaoka came up with a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged center. This was almost 20 years before Bohr came up with his planetary model. Unlike Bohr, Nagaoka thought that the atomic spectra should directly correlate to the electron's orbit in terms of frequency.

Albert Einstein

Jun 2, 1905

Einstein mathematically proved the existence of atoms. Through his work modern physics was born and further development of the atomic theory was enabled.

Robert A. Millikan

Jun 2, 1909

Millikan conducted the oil drop experiment. He determined the unit charge of the electron in his oil drop experiment at the University of Chicago. This allowed him to calculate the mass of the electron and the positively charged atoms.

Ernest Rutherford

Jun 2, 1911

Rutherford conducted the gold foil experiment by using alpha particles as projectiles to shoot at the atoms in a piece of thin (0.00006 cm) gold foil . He established that the nucleus was a very dense, small, and positively charged particle in the center of the cell. He also concluded that the electrons were located outside the nucleus and that majority of the atom was empty space.

Henry Moseley

Jun 2, 1913

Moseley re-organized Mendeleev's periodic table by number of protons. He did this by measuring the wavelengths of the x-rays given off by certain metals. This way he was able to find the number of positive charges in the nucleus of each atom.

Niels Bohr

Sep 1, 1922

Bohr's greatly contributed to the atomic theory with the Bohr model. It resembled a solar system with a positively charged nucleus and electrons in orbits. Bohr discovered that electrons traveled in different orbits and that the number of electrons in the outer orbit determines the properties of the atom.

Louis-Victor de Broglie

Jun 2, 1923

De Broglie devised wave-particle duality which stated that all matter had wave like properties to a certain extent. It helped to further establish Bohr's model.

Erwin Schrodinger

Jun 2, 1926

Schrodinger utilized quantum mechanics to create his interpretation of an atom, the cloud model. This is what modern scientists use as well. He explained the movement of an electron as a wave.

James Chadwick

Jul 2, 1932

James Chadwick experimented with beryllium atoms by bombarding them with alpha particles. An unknown radiation with that was neutral in charge was produced. this led to the discovery of the proton, a neutrally charged particle with the approximate mass of a proton. After this discovery, more adequate models of the atom became available to chemists.

Lisa Meitner, Otto Hahn, & Fritz Straussman

Jun 2, 1938

These three scientists performed experiments and managed to split Uranium atoms in a process called fission. This lead to further studies in the structure of atoms.

Enrico Fermi

Jun 2, 1942

Fermi conducted the first controlled chain reaction experiment by releasing energy from the atoms nucleus. He developed a self sustaining control of nuclear energy.

Max Planck

Jun 2, 1990

used the idea of quanta (discrete units of energy) to explain hot glowing matter. He also was the founder of quantum theory which is still used today to help develop atomic theory.