-
390 BCE
Democritus (460-390 BC)
Democritus would later develop the first theories about the atom. Democritus believed that everything was made up of tiny indestructible particles called atoms. His theories agreed with later scientific discoveries, but they did not follow the scientific method or explain chemical behavior. -
John Dalton (1766-1844)
Dalton’s theory was that all matter is made of atoms and that atoms are invisible and indestructible. He also stated that all atoms of a given element are identical in mass and properties. Also that compounds are formed by a combination of two or more different kinds of atoms. -
Antoine Lavoiser (1803)
The first breakthrough in the study of chemical reactions resulted from the work of the French chemist Antoine Lavoisier. He found that mass is conserved in a chemical reaction. The total mass of the products of a chemical reaction is always the same as the total mass of the starting materials consumed in the reaction. His results led to one of the fundamental laws of chemical behavior: the law of conservation of matter, which states that matter is conserved in a chemical reaction. -
Dmitri Mendeleev (1829-1869)
Dmitri Mendeleev published his periodic table of elements in 1869. His table arranged the known elements according to their chemical properties and in order of their relative atomic mass. This created several gaps in the periodic table and allowed him to predict eight other undiscovered elements correctly -
J.J. Thomson (1856-1940)
J. J. Thomson's atomic theory described the inner structure of atoms. According to Encyclopedia Brittanica, the Thomson atomic theory stated that atoms are uniform spheres of positively charged matter with electrons embedded. The theory was proposed around 1900 by Lord Kelvin and was heavily supported by Sir Joseph John Thompson. -
Max Planck (1858-1947)
He used the idea of quanta (discrete units of energy) to explain hot glowing matter.
-The electron with its negligible mass behaves like a photon in its revolution and is called photo electron.
-Thus every electron in an atom has its own discrete energy and moves around the nucleus. If external energy is supplied to an electron it gets excited and goes to a higher orbit.
-When it returns to its normal state, it releases the absorbed energy. -
Robert Millikan (1868-1953)
He carried out an experiments to find the quantity of charge carried by an electron. By using this value and the charge-to-mass ratio of an electron measured by Thomson, he calculated the mass of the electron. Millikan's values for the electron charge and mass are very closer to those we accept today. One of his famous experiments was the oil drop experiment, which helped to quantify the charge of an electron. -
Eugen Goldstein (1886)
Goldstein contributed greatly to the study of cathode rays. He discovered protons with the experiments he did with cathode rays which would knock electrons of atoms and attract them to a positively charged electrode. -
Henry Moseley (1887-1915)
In 1913 Moseley determined an atomic number for each known element and created a periodic table including these numbers. His discoveries showed that atoms of an element are always one proton heavier than the preceding element, but the neutrons of successive elements aren’t always exact . -
Ernest Rutherford (1911)
Rutherford overturned Thomson's model in 1911 with his well-known gold foil experiment in which he demonstrated that the atom has a tiny, heavy nucleus. Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure. Rutherford’s atomic theory was that an atom had a central positive nucleus with negative electrons orbiting it. He developed this theory with his gold foil experiment. -
Werner Heisenberg (1925)
Werner Heisenberg was a German theoretical physicist who made foundational contributions to quantum theory. He is best known for the development of the matrix mechanics formulation of quantum mechanics in 1925 and for asserting the uncertainty principle in 1926, although he also made important contributions to nuclear physics, quantum field theory and particle physics -
Erwin Schrodinger (1926)
Erwin developed mathematical equations to describe the motion and behavior of electrons in a hydrogen atom. His work. Lead to the electron cloud model. The probability of finding an electron is higher in the denser regions of the clouds in his model.
-Picture:
(Where there is a more dense area of dots is where you will have the higher probability of finding an electron rather than looking around the outside of the haze.) -
James Chadwick 1932
Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron. -
Neil Bohr: (November 18,1962)
Introduced by Niels Bohr and Ernest Rutherford in 1913, the model explains the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus( which is similar in structure to the solar system), but with attraction provided by electrostatic forces rather than gravity.
