History of Atoms- Mallory Bores

Democritus

400 BCE

The theory of Democritus suggested that everything is composed of atoms, which are physically, but not geometrically, indivisible. Between the atoms lies empty space. They're indestructible and always in motion. Democritus introduced the first idea of an atom. He contributed to and has changed the world forever with this discovery.

John Dalton

1803

Dalton suggested that elements consisted of atoms that were identical and had the same mass and that compounds were atoms from different elements combined together. New experiments with gas led Dalton to these discoveries.

Dmitri Mendeleev

1871

Mendeleev realized that both the physical and chemical properties of elements were related to their atomic mass in a certain way, periodically. He then arranged them where elements with similar properties would be near each other on his table, now known as the periodic table. This discovery is very obviously still relevant today and will be forever. He shaped chemistry into what we know now.

Eugene Goldstein

1885

Goldstein discovered that protons have equal and opposite charges. He used cathodes rays, but then later discovered canal rays. He is credited for the discovery of protons.

J.J Thomson

1897

Throughout Thomson's experiments with cathode-ray tubes, he discovered that all atoms contain tiny negatively charged subatomic particles or electrons.

Robert Millikan

1909

Millikan's first major finding was the accurate determining of the charge of an electron. He also found that the quantity was the same for all atoms, introducing the atomic structure of electricity.

Ernest Rutherford

1911

Rutherford constructed the nuclear structure of the atom, discovered alpha and beta rays, and proposed the laws of radioactive decay. He was the scientist who discovered Thomson's idea of the atom was incorrect. Rutherford won a Nobel Peace Prize for his work.

Niels Bohr

1913

Bohr proposed the theory for the hydrogen atom in 1913, this was based on the quantum theory where energy is only transferred in certain quantities. Electrons should move around the nucleus, but only in orbits.

James Chadwick

1932

Everyone knew at this point that the mass of an atom was mainly in the nucleus, Chadwick discovered another mass, the core, also contained a new uncharged particle. This was the neutron.