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460 BCE
Democritus
Democritus knew that if a stone was divided in half, the two halves would have essentially the same properties as the whole. He called these small pieces of matter "atomos". Democritus, theorized that atoms were specific to the material which they composed. In addition, Democritus believed that the atoms differed in size and shape, were in constant motion in a void, collided with each other; and during these collisions, could rebound or stick together. -
384 BCE
Aristotle
Aristotle did not believe in the atomic theory and he taught otherwise. He thought that all materials on Earth were not made of atoms, but of the four elements, Earth, Fire, Water, and Air. He believed all substances were made of small amounts of these four elements of matter. Aristotle's view was finally proven incorrect and his teachings are not present in the modern view of the atom. -
Mar 14, 1494
George Bauer
Presented the first scientific classification of minerals (based on their physical properties) and described many new minerals, their occurrence and mutual relationships. -
Robert Boyle
Boyle mainly studied gases. He discussed the possibility of atoms existing, however his work was greatly impeded by the church. He attempted alchemy or turning regular metals into gold. He made gas chambers to study from. He proposed that elements are composed of 'corpuscles' of various types and sizes that are able to organize themselves into groups that represent different chemical substances. He also was able to distinguish between a mixture and a compound. -
Antoine Lavoisier
Lavoisier proposed a definition of element - indivisible particles which we have found no means of separating. He demonstrated the formation of water in a sealed container by combining hydrogen and oxygen, showing water to be a compound instead of an indivisible element. -
John Daton
Democritus first suggested the existence of the atom but it took almost two millennia before the atom was placed on a solid foothold as a fundamental chemical object by John Dalton (1766-1844). Although two centuries old, Dalton's atomic theory remains valid in modern chemical thought. 1) All matter is made of atoms. -
Joseph Proust
Proust stated the Law of Definite Proportions. The law states that the ratio of elements in a compound is always the same/constant. He believed that matter could be put together in certain patterns to make bigger, different, unique matter. He set it up for Dalton to create the Law of Multiple Proportions and ultimately his Atomic Theory. -
Joseph Gay-Lussac
Law of combining volumes in 1808 – at the same temperature and pressure, two volumes of hydrogen gas react with one volume of oxygen gas to produce two volumes of water. Found that if you take a specific volume of gas and raise the pressure, the temperature increases. Gay-Lussac never worked with the atomic theory, but his combinations of gases were a great advance. -
Amedeo Avogadro
At the same temperature and pressure, equal volumes of gases contain the same number of molecules or atoms. When combined with Gay-Lussac's Law of Combining Volumes, the only possible formulas for hydrogen, oxygen and water are H2, O2 and H2O. Radical statement, not widely accepted until fifty years later. Avogadro also astutely reasoned that simple gases were not formed of solitary atoms but were instead compound molecules of two or more atoms. -
J.J. Thomson
J.J. Thomson discovered electrons and noticed that an atom can be divided. Also, he concluded atoms are made of positive cores and negatively charged particles within it. He developed the Plum Pudding Model before the atomic nucleus was discovered. -
Robert Millikan
The Millikan Oil Drop Experiment. An experiment performed by Robert Millikan in 1909 determined the size of the charge on an electron. He also determined that there was a smallest 'unit' charge, or that charge is 'quantized'. He received the Nobel Prize for his work. -
Ernest Rutherford
He discovered alpha and beta rays, set forth the laws of radioactive decay, and identified alpha particles as helium nuclei. 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. -
Niels Bohr
The Bohr model shows the atom as a small, positively charged nucleus surrounded by orbiting electrons. A stylized representation of a lithium atom illustrates his atomic model, that an atom is a small, positively charged nucleus surrounded by orbiting electrons. He made foundational contributions to understanding atomic structure and quantum theory, & he received the Nobel Prize in Physics in 1922. He made important contributions to the understanding of the structure and energy of the atom. -
Erwin Schrodinger
A powerful model of the atom was developed by Erwin Schrödinger in 1926. Schrödinger combined the equations for the behavior of waves with the de Broglie equation to generate a mathematical model for the distribution of electrons in an atom. His great discovery, Schrödinger's wave equation, was made at the end of this epoch-during the first half of 1926. His belief that atomic spectra should really be determined by some kind of eigenvalue problem. -
Henri Becquerel
Worked with properties of the atom, such as magnetism and radioactivity. Found that some atoms have natural radioactivity. His discovery of radioactivity allowed later scientists to perfect the atomic model. -
Max Planck
Quantum Theory; energy is given off in little packets; energy in wave form is restricted to specific quantities, and led to understanding of atom levels and advancement in the atomic model. -
Louis de Broglie
Discovered that electrons had a dual nature-similar to both particles and waves. Particle/wave duality. -
Werner Heisenberg
Described atoms by means of formula connected to the frequencies of spectral lines. Proposed Principle of Indeterminancy - you cannot know both the position and velocity of a particle.
