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427 BCE
Plato
Plato introduced the atomic theory in which ideal geometric forms serve as atoms, according to which atoms broke down mathematically into triangles, such that the form elements had the following shape: fire (tetrahedron), air (octahedron), water (icosahedron), earth (cube). -
400 BCE
Democritus
Democritus believed that atoms were uniform, solid, hard, incompressible, and indestructible and that they moved in infinite numbers through empty space until stopped. Differences in atomic shape and size determined the various properties of matter. -
332 BCE
Aristotle
In Aristotle's time, atomists held that matter was fundamentally constructed out of atoms. These atoms were indivisible and uniform, of various sizes and shapes, and capable only of change in respect of position and motion, but not intrinsic qualities. -
Lavoisier
Antoine Lavoisier's work in defining the law of conservation of mass would help to shape atomic theory. This discovery was influential in atomic theory because it defined that matter was composed of atoms that were not created or destroyed during chemical reactions. -
John Dalton
Dalton proposed that every single atom of an element, such as gold, is the same as every other atom of that element. He also noted that the atoms of one element differ from the atoms of all other elements. Today, we still know this to be mostly true. -
The Alchemists
The alchemists believed that all metals were formed from two principles — mercury and sulfur. The mercury, with its essential property of fluidity and fusibility, gave rise to the malleability of metals. The sulphur, with its essential property of combustibility, contributed body and calcination (rusting). -
Newlands Law of Octaves
Newlands Law of Octave states that when Elements are put in increasing order of Atomic Mass, the properties of every eighth Element starting from any Element are a repetition of the properties of the starting Element. -
Mendeleev's Pd. Table
Mendeleev arranged the elements in order of increasing weight and broke them into rows such that elements in each column shared valence, the number of other atoms they combined with, as well as other properties. -
Photoelectric Effect
The photoelectric effect, it is a phenomenon in which electrically charged particles are released from or within a material when it absorbs electromagnetic radiation. The effect is often defined as the ejection of electrons from a metal plate when light falls on it. The photoelectric effect was discovered in 1887 by the German physicist Heinrich Rudolf Hertz. -
Discovery of Radioactivity
Henri Becquerel Discovers Radioactivity. In one of the most well-known accidental discoveries in the history of physics, French physicist Henri Becquerel opened a drawer and discovered spontaneous radioactivity. -
Discovery of the electron
During the 1880s and '90s scientists searched cathode rays for the carrier of the electrical properties in matter. Their work culminated in the discovery by English physicist J.J. Thomson of the electron in 1897. -
Plancks Quantum Theory of Light
Specifically, Planck's quantum theory of fussy light tells us that light bulb filaments should be heated to a temperature of about 3,200 Kelvin to ensure that most of the energy is emitted as visible waves. Much hotter, and we'd start tanning from the ultraviolet light. -
Plum Pudding Model
The plum pudding model (also known as Thomson's plum pudding model) is a historical scientific model of the atom. The plum pudding model is defined by electrons surrounded by a volume of positive charge, like negatively-charged “plums” embedded in a positively-charged “pudding” (hence the name). -
Rutherfords Gold Foil Experiment
Rutherfords Gold Foil Experiment took place in. A piece of gold foil was hit with alpha particles, which have a positive charge. Most alpha particles went right through. This showed that the gold atoms were mostly empty space. -
Bohr's Planetary Model
He believed the electrons encircle the nucleus of the atom in specific allowable paths called orbits. -
Robert Millikan
In 1910, Robert Millikan discovered the charge of an individual electron. He published his results in 1913. Millikan also made the discovery that all electrons carry the same amount of negative charge. -
Mosleys Atomic Numbers
In 1914 Moseley published a paper in which he concluded that the atomic number is the number of positive charges in the atomic nucleus. He also stated that there were three unknown elements, with atomic numbers 43, 61, and 75, between aluminum and gold. (There are, in fact, four. -
Discovery of the proton
The proton was discovered by a man named Ernest Rutherford in the early 1900's. During this period, his research resulted in a nuclear reaction which led to the first 'splitting' of the atom, where he discovered protons. He named his discovery “protons” based on the Greek word “protos” which means first. -
Charge of the Electron
electron, lightest stable subatomic particle known. It carries a negative charge of 1.602176634 × 10−19 coulomb, which is considered the basic unit of electric charge. The rest mass of the electron is 9.1093837015 × 10−31 kg, which is only 1/1,836the mass of a proton. -
Schrodinger Equation
The Schrödinger equation is a linear partial differential equation that governs the wave function of a quantum-mechanical system. It is a key result in quantum mechanics, and its discovery was a significant landmark in the development of the subject. -
Heisenberg Uncertainty Principle
Formulated by the German physicist and Nobel laureate Werner Heisenberg in 1927, the uncertainty principle states that we cannot know both the position and speed of a particle, such as a photon or electron, with perfect accuracy; the more we nail down the particle's position, the less we know about its speed and vice. -
Discovery of the Neutron
In May 1932 James Chadwick announced that the core also contained a new uncharged particle, which he called the neutron. Chadwick was born in1891 in Manchester, England.
