Evolution of Atomic Theory

Democritus

465 BCE

Democritus created the first atomic theory:
1.All matter consists of invisible particles called atoms.
2. Atoms are indestructible.
3. Atoms are solid but invisible.
4. Atoms are homogenous.
5. Atoms differ in size, shape, mass, position, and arrangement.
->Solids are made of small, pointy atoms.
->Liquids are made of large, round atoms.
->Oils are made of very fine, small atoms that can easily slip past each other.

Aristotle

335 BCE

Aristotle believed that instead of everything being comprised of atoms, everything was comprised of four basic elements; earth, fire, water, and air. Unfortunately more people believed Aristotle’s theory than Democritus. Aristotle thought that these four basic elements combined in certain ways made up everything. He taught this from 335 B.C.E until 323 B.C.E

John Dalton

1800

Dalton was one of the first who proposed a basic theory about atoms. His theory is still holds weight today. His theory is as follows:
1) All matter is made of atoms. Atoms are indivisible and indestructible. 2) All atoms of a given 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.

Eugene Goldstein

1886

Using a cathode ray tube, Goldstein noticed that there were not just electron waves traveling within the tubes. There was another type of particals, moving the opposite way. Since it was moving the opposite way it must be positively charged, otherwise known as a proton. This was troubling to Goldstein as he could not explain it. As a result it took 12 years for his paper to be published.

Joseph John Thomson

1897

JJ Thomson is the man who first discovered the electron as well of isotopes. Before him people believed the atom could simply not be broken down further. JJ Thomson said that negatively charged electrons were help in a positively charged “soup”. This is known as the plum-pudding model (which is now outdated). He did this by using a Cathode Ray Tube

Max Plank

1900

Planck revolutionized quantum theory. He theorized that energy, instead of being a constant wave, is released in packets. He also created Plancks constant, an energy constant which describes how much is released per one quantum. Einstein later used this is his theories.

A Transition Into Post-Quantum Era

1900

Everyone from Max Planck and onward are now in the Post-Quantum era. Everyone before this date is apart of the Pre-Quantum era

Robert Millikan

1909

Robert Millikan was an American physicist who along with many other achievements discovered the charge of an electron through a famous oil experiment. He did this by dropping oil through a charged metal plate, he found out that their electric force was equal to the gravitation force, thus the more charged, the slower they would fall.

Ernst Rutherford

1910

Ernst Rutherford is the genius behind the famous gold foil experiment. By shooting alpha particals at a thin (extremely thin) layer of gold. If the alpha particals bounced off, went through, or their path changed, he was able to discover that a nucleus layed in the middle of an atom, surrounded by electrons.

Niels Bohr

1913

Without a doubt Niels Bohr is one of if not the most important person on this list. He proposed the planetary model, where electrons orbited the nucleus like our solar system. This is the model we still use today.

Erwin Schrödinger

1926

Erwin Schrödinger said that electrons orbited a nucleus like a cloud. This is the same model we used today. Using math he predicted the likelihood of finding a electron in a certain part of the cloud. He also defined the concept of sub-energy levels. This particular model became known as the quantum mechanical model and is the basis of today’s model.

Louis de Brogile

1929

Louis de Brogile suggested that electrons functioned in waves. While at first ignored by the scientific community once Einstein discovered this Brogile’s theory gained traction and was proven.

James Chadwick

1932

James Chadwick is credited with the discover of the neutron. After WWI Chadwick returned home and discovered the Proton within the nucleus of an atom, however his team soon realized their was something else too. The atomic number (or number of protons) didn’t match with how much an atom weighed (atomic mass). Although their was no evidence at first but after launching alpha particles at Berryllium he discovered both Protons and Nuetrons.

Werner Heisenberg

1981

Werner Heisenberg created the uncertainty principal. This is to say that a electrons position and velocity can never be known as if you tried to observe it the photons would cause the electron to change its position and velocity due to electrons being so small and light. This is why an electron had never been directly observed.

Works Cited

2017

"All Nobel Prizes in Physics." Nobelprize.org, www.nobelprize.org/nobel_prizes/physics/laureates/index.html.
"Development of the Atomic Theory." www.abcte.org/files/previews/chemistry/s1_p6.html.
"Electronic Structure of Atoms | Chemistry | Science |Khan Academy." Khan Academy, www.khanacademy.org/science/chemistry/electronic-structure-of-atoms#history-of-atomic-structure.

Works Cited

2017

"Molecular Expressions: Science, Optics and You - Timeline - Pioneers in Optics." Molecular Expressions: Images from the Microscope, micro.magnet.fsu.edu/optics/timeline/people/index.html.
"A Science Odyssey: People and Discoveries: People." PBS: Public Broadcasting Service, www.pbs.org/wgbh/aso/databank/bioindex.html.