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400
Democritus
Democritus lived around the time of 400 B.C. He was a philosopher and scientist who theorized that all matter was made of individual particles. His theory was based upon logical reasoning because experiments were not considered necessary at this time. -
Period: 400 to
Atomic Timeline
A timeline of the history of important discoveries regarding the study of the atom. -
Antoine Lavoisier
Antoine is known his law of conservation of mass. This states that the mass of a closed system must remain constant over time. System mass cannot change quantity if it is not added or removed. This law implies that mass cannot be created or destroyed, but can be rearranged or transferred from one object to another. -
John Dalton
Dalton created the spherical model and the first atomic theory. This theory stated that elements are made of atoms, atoms of the same element are identical, while atoms of a different element differ in properties, and that atoms cannot be created, combined, or destroyed. Also that atoms of different elements form in simple, whole number ratios to form chemical compunds, and that atoms are combined in chemical reactions. -
Joseph Proust
In 1812 Joseph Prout created his law of definite proportions. This law stated that a chemical compound always conatins the same proportion of elements by mass. -
Eugen Goldstein
It is often argued whether Goldstein or Rutherford is the true founder of the proton. His discovery is said to have occured when he observed the negatively charged cathode moving toward the positively charged anode. While still debateable, this later was recognized as the discovery of the proton. -
Antoine Henri Becquerel
Becquerel is most famous for his discovery of radioactivity. This discovery occured when Becquerel left a piece of uranium crystal out in the sun and then placed it onto a photographic plate. He returned to find that the crystal had left a perfect image of itself on the plate. Becquerel placed both objects in a drawer and after several days passed removed them to find that the crystal had continued to trace itself even in the absence of light. He had just discovered radioactivity. -
J.J. Thomson
During his experiment with cathode rays, Thomson proved that they were made up of a negatively charged particle, thus discovering the electron. To exemplify his discovery, Thompson designed the plum pudding model which showed electron as plums, floating in a positively charged "pudding" which balanced out the atom. -
Ernest Rutherford
Rutherford worked with helium ions and made significant contributions to the field of radioactivity. He designed and carried out the gold foil experiment which proved the structure of the atom. This lead to his creation of the nuclear model which included a very small, very dense, positively charged nucleus surrounded by a field of electrons. -
Niels Bohr
Niels Bohr found that electrons move around the nucleus in orbits at different energy levels. Electrons can then jump to the different energy levels. Additionally, he developed a model called the Bohr model which showed that electrons that orbit the nucleus have a specific size and energy, and that the amount of energy in each orbit is related to its size. -
Max Planck
Planck is well known for originating quantum theory, which provided a mathmatical description of the dual particle-like and wave-like behavior and interactions of energy and matter. -
Erwin Schrodinger
schrodinger helped discover results in the field of wave mechanics. He also developed the Schrodinger equation to calculate stationary and time dependent waves, and further enhanced the quantum theory. Additionally his is known for inventing the quantum mechanic model, that uses orbitals and volumes of space to show where there is likely to be an electron. -
Werner Heisenburg
Heisenburg created the uncertainty principle which put limits on the accuracy of knowledge regarding atomic behavior. This theory stated that it is impossible to exactly measure the velocity and position of an object at the same time. -
James Chadwick
Chadwick is most well known for his discovery of the neutron. During Chadwick's time there was speculation that a third subatomic particle existed, however scientific proof was difficult to find. After studying works in Europe, he collected proof of the neutron, found its mass and published his findings. This discovery then lead to Chadwick's modified nuclear model. The model includes a nucleus with positively charged protons, and neutral neutrons.
