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427 BCE
Plato
Plato had a virtue-based eudaemonistic conception of ethics. He introduced the atomic theory in which ideal geometric forms serve as atoms, according to which atoms broke down mathematically into triangles. -
400 BCE
Democritus
The ancient atomic theory was proposed in the 5th century bce by the Greek philosophers Leucippus and Democritus. Democritus believed that atoms were uniform, solid, hard, incompressible, and indestructible and that they moved in infinite numbers through empty space until stopped. -
332 BCE
Aristotle
Aristotle did not believe in the atomic theory and he taught so 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. -
Lavoisier
Lavoisier's atomic theory model was grounded in the law of conservation of mass explaining matter was conserved during chemical changes. His experiments also shaped the idea that matter was composed of chemical compounds. In 1789 he published the first periodic table, his findings describing the law of conservation of mass, and discovered that various types of matter were composed of chemical compounds. -
John Dalton
John Dalton has a theory that states that all matter is made of atoms, which are indivisible. The second part of the theory says all atoms of a given element are identical in mass and properties. The third part says compounds are combinations of two or more different types of atoms. -
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 rusting. -
Newlands Law of Octaves
Describes that when Elements are arranged 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 made a periodic table and 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 is a phenomenon in which electrically charged particles are released from or within a material when it absorbs electromagnetic radiation. It was first discovered in 1887 by German physicist Heinrich Rudolf Hertz while he was working on relevant radio waves. -
Discovery of the electron
Joseph John Thomson is widely recognized as the discoverer of the electron. He was the Cavendish professor of Experimental Physics at Cambridge University and director of its Cavendish Laboratory from 1884 until 1919. They were discovered through Thomson's experiments with cathode ray tubes. -
Discovery of Radioactivity
French physicists Pierre and Marie Curie discovered the strongly radioactive elements polonium and radium, which occur naturally in uranium minerals. In their lab they studied pitchblende, in which uranium is the primary element and reported the probable existence of one or more other radioactive elements in the mineral. -
Planck's Quantum Theory of Light
This theory 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. It is also stated that different atoms and molecules can emit or absorb energy in discrete quantities only. -
Plum Pudding Model
The plum pudding model (also known as Thomson's plum pudding model) is a scientific model of the atom. It is defined by electrons surrounded by a volume of positive charge, like negatively-charged “plums” embedded in a positively-charged “pudding” (hence the name). -
Albert Einstein
In 1905, Einstein mathematically proved the existence of atoms, and thus helped revolutionize all the sciences through the use of statistics and probability. Atomic theory says that any liquid is made up of molecules which were invisible at the time. Furthermore, these molecules are always in random, ceaseless motion. -
Rutherford's Gold Foil Experiment
The Rutherford gold-foil experiment is when a piece of gold foil is 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. -
Charge of the electron
Electrons have an electric charge of −1 and their mass is approximately about 1/2000 the mass of a neutron or proton. Electron charge is usually denoted by the symbol e. It is a fundamental physical constant that is used to express the naturally occurring unit of electric charge which is = 1.602 × 10-19 coulomb. -
Bohr's Planetary Model
This model is a structural example of an atom proposed by Neil Bohr. It shows the electrons moving around the nucleus in a distinct circular pattern. -
Robert Millikan
Millikan's work demonstrated that electrons did have a discrete, quantifiable charge. His "oil drop experiment" used a simple apparatus in which he balanced the actions of electric, gravitational, and air drag forces. Using the apparatus, he was able to calculate the charge on an electron. -
Mosley's Atomic Numbers
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. -
Discovery of the proton
Earnest Rutherford discovered protons when his research resulted in a nuclear reaction which led to the first 'splitting' of the atom. He named his discovery “protons” based on the Greek word “protos” which means first. -
Schrodinger Equation
Schrodinger's equation, "H ^ ψ = E ψ" can be solved to yield a series of wave function, each of which is associated with an electron binding energy. -
Heisenberg Uncertainty Principle
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. It was formulated by the German physicist and Nobel laureate Werner Heisenberg in 1927. -
Discovery of the Neutron
It was announced in 1932 by James Chadwick that the core also contained a new uncharged particle, which he called the neutron. The physicist conducted an experiment in which he bombarded Beryllium with alpha particles from the natural radioactive decay of Polonium. The resulting radiation showed high penetration through a lead shield, which could not be explained via the particles known at that time.