Atomic Theory timeline

Democritus

460 BCE

Democritus was a Greek philosopher and the first person in common knowledge to propose the idea of atoms. He believed that atoms were indivisible, which we know is not true today. This is why he named them atomos, which means "not to cut" in english. Duignan, Brian. “Democritus.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., https://www.britannica.com/biography/Democritus.

Aristotle

350 BCE

Aristotle proposed that all matter came back to 4 elements: wind, water, air, and earth instead of Democritus' theory of particles. “Sutori.” Sutori, https://www.sutori.com/item/aristotle-s-views-on-atoms-450-b-c-aristotle-did-not-believe-in-the-atomic-theo.

Joseph Proust

1784

Joseph Proust created the "Law of Definite Proportions". It states that any amount of one element has the same ratio proportions no matter where it is found. Atomic Theory: Joseph Proust, https://staff.concord.org/~ddamelin/chemsite/c_AtomicTheory/Proust.html.

John Dalton

1803

John Dalton created the first atomic model called "The Atomic Theory of Matter". This theory proposed that each type of matter contains only one type of atom. He also proposed that each type of atom could be categorized as an element. “Atomic Model.” BrainPOP, https://www.brainpop.com/science/matterandchemistry/atomicmodel

Dmitri Mendeleev

1869

Dmitri Mendeleev created Periodic Table. Elements on this table were arranged by atomic weight. He according to this table, he guessed correctly some of the properties of unknown elements. Bensaude-Vincent, Bernadette. “Dmitri Mendeleev.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., https://www.britannica.com/biography/Dmitri-Mendeleev/.

J.J. Thomas

Apr 30, 1897

J.J. Thomas discovered the electron using a cathode rays. He also proposed the "plum-pudding" atomic model. This model had electrons scattered in a positive sphere. “The Nobel Prize in Physics 1906.” NobelPrize.org, https://www.nobelprize.org/prizes/physics/1906/thomson/biographical/.

Ernest Rutherford

1907

Ernest Rutherford was taught by J.J. Thomson. He proved J.J. Thomson's theory wrong with this gold foil experiment. Rutherford’s hypothesis was that alpha ray beam would go straight through the gold foil but that was not the case. He then theorized that the atoms have a small solid nucleus made of protons, and the rest was mostly empty space and electrons. “Ernest Rutherford.” Biography.com, A&E Networks Television, 25 June 2019, https://www.biography.com/scientist/ernest-rutherford.

Robert Millikan

1909

Robert Millikan determined the charge of an electron by measuring the course of charged water droplets in an electric field. “The Nobel Prize in Physics 1923.” NobelPrize.org, https://www.nobelprize.org/prizes/physics/1923/millikan/biographical/

Frederick Soddy

1913

Frederick Soddy created the concept of isotopes. Isotopes are types of an element that have different amounts of neutrons but have the same atomic masses. “The Nobel Prize in Chemistry 1921.” NobelPrize.org, https://www.nobelprize.org/prizes/chemistry/1921/soddy/biographical/.

Niels Bohr

1913

Niels Bohr created an atomic model that is now know as the Bohr model. He was the first to propose that electrons have orbit around the nucleus in the atom. The only flaw with his model is that it is based around the element hydrogen, which only has 1 electron. This becomes a problem when you get up to elements like Uranium, which has 92 electrons. “The Nobel Prize in Physics 1922.” NobelPrize.org, https://www.nobelprize.org/prizes/physics/1922/bohr/biographical/.

Henry Moseley

1913

Henry Moseley determined that all atoms of the same element have the same number of protons in the nucleus by studying the X-ray spectra of elements. This replaced Dmitri Mendeleev's method of labeling elements on a periodic table. Britannica, The Editors of Encyclopaedia. “Henry Moseley.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 6 Aug. 2019, https://www.britannica.com/biography/Henry-Moseley.

Erwin Schrödinger

1925

Erwin Schrödinger used equations to see the likely hood of a electron to be in one place. “How Did Erwin Schrödinger Contribute to the Atomic Model?” Quora, https://www.quora.com/How-did-Erwin-Schrödinger-contribute-to-the-atomic-model.

Werner Heisenberg

1925

Werner Heisenberg contributed to the atomic theory by creating quantum mechanics and the Uncertainty Principle using Erwin Schrödinger Equation. This principle states that all you can not know both the momentum and the position of a particle. Heisenberg published his uncertainty principle and theory of quantum mechanics in 1925. Beyler, Richard. “Werner Heisenberg.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 30 Aug. 2019, https://www.britannica.com/biography/Werner-Heisenberg.

James Chadwick

1932

James Chadwick discovered the neutron in 1932 under Rutherford. The discovery proses also lead the paved the ground works of atomic bombs. “The Nobel Prize in Physics 1935.” NobelPrize.org, https://www.nobelprize.org/prizes/physics/1935/chadwick/biographical/.

Murray Gell-Mann

1964

Murray Gell-Mann theorized that protons and neutrons are made of something even smaller-quarks. Quarks can not be directly observed. This theory was latter proved correct due to experimentation “The Nobel Prize in Physics 1969.” NobelPrize.org, https://www.nobelprize.org/prizes/physics/1969/gell-mann/biographical/.