The Atom

Democritus

400 BCE

460-370 BC
Democritus was the first philosopher to propose that matter wasn’t infinitely divisible. He believed matter was made of atomos. He also believed that different kinds of atoms had different sizes and shapes, and these factors determined the properties of the matter. His idea was that atoms are solid, homogeneous, indestructible, and indivisible. Democritus was often criticized for his ideas by other philosophers. They wondered how atoms are held together, which he couldn’t answer.

John Dalton

1800

1778-1844
Dalton was a school teacher in England.He believed in Democritus’ ideas about the atom, making their ideas very similar.He studied many chemical reactions, making observations and measurements.His results of his research are known as Daltons atomic theory, which he proposed in 1803.This theory states that matter is composed of atoms,and these atoms combine in simple whole-number ratios to form compounds.Dalton published all of his ideas in his book,A New System of Chemical Philosophy.

Dmitri Mendeleev

1869

1834-1907
Dmitri Mendeleev is best known for inventing the periodic table and formulating the periodic law around 1869. He created the symbols for all the elements (that were known at the time) and put them in order according to there atomic weight/number.

J.J. Thompson

1897

1856-1940
J.J. Thompson was credited with finding the first subatomic particle, the electron, on April 30, 1897. He also stated that atoms were made of smaller components. He conducted the cathode ray tube experiment which helped prove the existence of electrons in the atom. Thompson’s model of the atom is the Plum Pudding Model, which depicts electrons in a sea of positive charge. Although, his model was accepted, it did not stick around for long since more developed models were along the way.

Albert Einstein

1905

1879–1955
Albert Einstein is known for many things in the scientific world, but he was the scientist that finally proved the existence of the atom by his Pollen Grain Experiment. This experiment was simple: dropping pollen grain into water and watch what the grain does. Instead of staying still at the top of the water, the pollen grain danced around the water. Einstein explained that the pollen could not move around without the existence of atoms.

Ernest Rutherford

1911

1871–1937
Rutherford’s main contribution towards the atom was his discovery of the nucleus. In 1911, Rutherford experimented with alpha particles and gold foil. After most of the alpha particles passed the gold foil with no deflection, he concluded that the atom was mostly empty space. This proved Thompson’s plum pudding model false. Instead of positive charge being distributed throughout the atom, he reasoned that the positive charge, and most of the mass, was found at the center—the nucleus.

Niels Bohr

1913

1885-1962
Niels Bohr was the physicist who developed one of the most important atomic models, The Bohr Model. In 1913, Bohr came up with a theory about the hydrogen atom based off the quantum theory. He stated the the electrons are limited to individual orbits around the nucleus. Bohr modified Rutherford’s planetary model of the atom by concluding that electrons are arranged based on size and energy levels. Radiation was given off from an atom when an electron jumped to a lower level.

Robert Millikan

1913

1868-1953
Robert Millikan is credited for determining the magnitude of an electrons charge. His experiment was the Oil Drop Experiment, which he used to discover the charge and mass of electrons. This experiment allowed him to determine the electric charge of each drop of oil. He also discovered that electrons are tiny, a lot smaller than the mass of atoms.

Louis de broglie

1923

1892-1987
De Broglie is best known for his “Recherches sur la Théorie des Quanta” (Research on Quantum Physics), which helped him gain his official doctor degree. In 1924, DeBroglie brought forth a new concept. He stated that subatomic particles, such as electrons, can also be defined and described as waves. His experiment of Electron diffraction by crystals brought him to this conclusion. His theories and research, serve as a imperative foundation for what is presently called “Wave Mechanics”.

Erwin Schrodinger

1926

1887-1961
Erwin Schrodinger was a theoretical physicist and scholar who came up with the wave function and equation for electron movements, which was groundbreaking in the world of science. He assumed that electrons could be regarded as both particles and waves, and formulated a wave equation that accurately calculated the energy levels of electrons in atoms, which he created in 1926.

Werner Heisenberg

1927

1901-1976
Werner Heisenberg is known for his Uncertainty Principle and Theory of Quantum Mechanics. He published this Principle and theory in 1925. The Heisenberg Uncertainty Principle states that it is impossible to determine simultaneously both the position and the velocity of a particle. Heisenberg says that we can not know both the energy and position of electrons.