Atomic Model Timeline

Democritus

442 BCE

In approximately 442 BCE, Democritus theorized that everything is composed of atoms. Democritus said that atoms were indestructible. He also thought that atoms were physically, but not geometrically divisible. He believed that atoms were always in motion with infinite number of atoms in multiple different forms.

Aristotle

340 BCE

Aristotle vehemently opposed and contradicted Democritus’s atomic theory, believing that there were no atoms. Instead Aristotle thought there were fundamental elements that made up the Earth: fire, air, water, and earth.

Antoine Lavoisier

1772

Lavoisier is known as the "father of modern chemistry". Lavoisier founded the Law of Conservation ion of Matter. He stated that the total ending mass of a product of a chemical reaction and the total starting mass was equal. (Matter would be conserved through a chemical reaction.)

Antoine Lavoisier

1772

This is an example of the Law of Conservation of Matter.

John Dalton

1803

Dalton studied Lavoisier's work in the chemistry field. Dalton had an atomic theory. The main points of his theory included:
-all atoms of one element are identical
-atoms make up everything
-atoms are indestructible
-atoms of different elements vary in size and mass
-compounds can only be combined by whole number atoms
-a chemical reaction rearranges the atoms in the reactant and product
Multiple of the points in Dalton's theory are similar to what Democritus hypothesis many centuries before.

Henri Becquerel

1896

Henri Becquerel was the first scientist to ever discover radioactivity in any element. Becquerel originally found radioactivity in uranium. Becquerel went on to earn the 1903 Nobel Prize for Physics, sharing the award with Marie and Pierre Curie, for his work about radioactivity. Radioactivity led to a further understanding of the atom and what makes up the atom.

J.J. Thomson

1897

In 1897, J.J. Thomson observed negatively charged particles within an atom. Never being recorded by scientist before, J.J. Thomson was the first scientist to encounter electrons. This was a monumental achievement because the electron is vital to the makeup and structure of an atom. Later scientists based their models off of Thomson’s discovery including Millikan, Rutherford, and many more.

Marie and Pierre Curie

1898

Marie and Pierre Curie studied the radioactivity within elements after Henri Becquerel led a breakthrough by discovering radioactivity. Marie and Pierre Curie conducted experiments, resulting in finding radioactivity in other elements than uranium. The Curies’s and Becquerel’s results refined the understanding of what properties an atom possessed.

Max Planck

1900

In 1900, Planck established the quantum theory of energy. The quantum theory of energy was significant in understanding the atom because it helped scientists to provide an explanation for the behavior and nature of energy on an atomic level, as well as subatomic level. Many scientist would later use this theory as a basis in their research, including Werner Heisenberg.

Robert Millikan

1909

In 1909, Millikan measured the size of the charge of an electron. Which allowed Millikan to discover that there existed a smallest unit of a charge. He went on to receive a Nobel Prize for Physics in 1923 for his work that helped advance the knowledge of the electron. Millikan’s experiment relates to the discovery of the electron by J.J. Thomson.
(Millikan used an oil drop experiment to determine the measurement of the charge of electrons.)

Ernest Rutherford

1911

Rutherford is renowned for recognizing that the atom contains a positively charged nucleus where the mass is concentrated, with electrons circling the nucleus through a gold foil experiment. What Rutherford found is called the Rutherford, and sometimes planetary, model. The Rutherford model was fundamental in laying out the basic foundations and placements of what an atom was made up of, advancing what was known about the atomic model. This model would later be refined by Niels Bohr.

Henry Moseley

1913

Henry Moseley was the scientist who discovered that the atomic number determines what the element is, not the atomic mass. This directly contradicted what earlier scientists had perceived as correct. Moseley also correlated the atomic number with the charge of the nucleus. This allowed the organization of the periodic table to be more accurate and laid a basis of elements for scientists later on.

Niels Bohr

1913

In 1913, Niels Bohr illustrated an atomic model based off Rutherford’s model and the fact that Bohr figured out that electrons orbit the nucleus in a prescribed circle. Bohr also stated that when electrons moved through the orbits a light quantum is emitted. Additionally, Bohr proposed the idea that for hydrogen atoms, energies can only be transferred in certain quantities.

Erwin Schrodinger

1926

In 1926, Erwin Schrodinger enhanced Bohr’s atomic model. By using mathematical equations, Schrodinger could locate the area where an electron was in an atom. Schrodinger also contradicted what scientists had previously believed. Scientists thought that the electrons moved in orbits around the nucleus, but Schrodinger stated that electrons were in a cloud around the nucleus.This is known as the quantum mechanical model, or the electron cloud model, which would later be expanded by Heisenberg.

Werner Heisenberg

1927

In 1927, Heisenberg founded quantum mechanics, extending Schrodinger’s researches on quantum mechanics and the atom. Heisenberg, mentored by Bohr, later published the Uncertainty Principle which stated that the momentum and position of a particle in an atom can not be determined at the same time, but rather a relation can be found in indeterminacies. Heisenberg’s quantum mechanics helped to improve the depiction of the atomic model.

James Chadwick

1932

James Chadwick presented the discovery of the an uncharged particle within the nucleus of an atom in 1932. Chadwick called the particle a neutron. Chadwick finding the neutron refines the atomic model from previous scientists because it was believed that there were only protons in the nucleus. However, the rest of Chadwick’s model was consistent with the former model by Schrodinger, the nucleus containing protons and an electron cloud around the nucleus.