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400 BCE
Democritus
Democritus was a key figure in the development of the atomic theory of the universe. He theorized that all material bodies are made up of indivisibly small “atoms.” -
Law of conservation of mass
The Law of Conservation of Mass dates from Antoine Lavoisier's 1789 discovery that mass is neither created nor destroyed in chemical reactions. In other words, the mass of any one element at the beginning of a reaction will equal the mass of that element at the end of the reaction. This Law coincides with john daltons atomic theory. -
John Dalton's Atomic Theory
Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass. The main points of Dalton's atomic theory are: Everything is composed of atoms, which are the indivisible building blocks of matter and cannot be destroyed. All atoms of an element are identical. The atoms of different elements vary in size and mass. -
The law of definite proportion
The law of definite proportion states that chemical compounds contain exactly the same proportion of elements by mass. ... The Dalton atomic theory explains the law of definite proportions. Dalton proposed that the smallest particle of carbon monoxide is a molecule that contains one oxygen atom and one carbon atom. -
Law of multiple proportions
The law was announced (1803) by the English chemist John Dalton, and its confirmation for a wide range of compounds served as the most powerful argument in support of Dalton's theory that matter consists of indivisible atoms. -
Dmitri Mendeleev
Arranged elements into 7 groups with similar properties. He discovered that the properties of elements "were periodic functions of their atomic weights". This became known as the Periodic Law. -
Henri Becquerel
Like Thomson's discovery of the electron, the discovery of radioactivity in uranium by French physicist Henri Becquerel in 1896 forced scientists to radically change their ideas about atomic structure. Radioactivity demonstrated that the atom was neither indivisible nor immutable. -
J.J. Thomson's Cathode Ray experiment
In 1897, J.J. Thomson discovered the electron by experimenting with a cathode ray, tube. He demonstrated that cathode rays were negatively charged. -
Marie Curie
Curie conducted experiments on uranium rays and discovered that they remained constant, no matter the condition or form of the uranium. The rays, she theorized, came from the element's atomic structure. This revolutionary idea created the field of atomic physics -
Plum Pudding Atomic Model
J.J. Thomson's plum pudding model has electrons surrounded by a volume of positive charge, like negatively charged "plums" embedded in a positively charged "pudding". -
Ernest Rutherford's Gold foil experiment
In 1911, he was the first to discover that atoms have a small charged nucleus surrounded by largely empty space, and are circled by tiny electrons, which became known as the Rutherford model of an atom. The gold foil experiment is the reason for his Nucleus theory. He stated that positive charge is not like pudding but concentrated in the nucleus -
Robert Millikan's Oil Drop experiment
His earliest major success was the accurate determination of the charge carried by an electron, using the elegant “falling-drop method”, or his "Oil Drop" experiment; he also proved that this quantity was a constant for all electrons (1910), thus demonstrating the atomic structure of electricity. -
Rutherford Model
The model described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun. -
Niels Bohr
In 1913, Niels Bohr proposed a theory for the hydrogen atom, based on quantum theory that some physical quantities only take discrete values. Electrons move around a nucleus, but only in prescribed orbits, and If electrons jump to a lower-energy orbit, the difference is sent out as radiation. -
Bohr Planetary Model
In atomic physics, the Bohr model or Rutherford–Bohr model, presented by Niels Bohr and Ernest Rutherford in 1913, is a system consisting of a small, dense nucleus surrounded by orbiting electrons—similar to the structure of the Solar System, but with attraction provided by electrostatic forces in place of gravity. -
Erwin Schrodinger's Quantum mechanical model
In 1926 Erwin Schrödinger, an Austrian physicist took the Bohr atom model one step further. Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom. -
James Chadwick
Chadwick is best known for his discovery of the neutron in 1932. A neutron is a particle with no electric charge that, along with positively charged protons, makes up an atom's nucleus. Bombarding elements with neutrons can succeed in penetrating and splitting nuclei, generating an enormous amount of energy. -
Murray Gell-Mann
Murray Gell-Mann started working at the Institute for Nuclear Studies, the University of Chicago in 1952, where he introduced the concept of “strangeness”, a quantum property and the force that holds the components of the atomic nucleus, in 1953.