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600 BCE
The Alchemists
The alchemists began examining the atomic theory about two centuries after the death of Aristotle. They used Aristotle’s idea about matter and began to create experiments and activities with them. By treating different metals and ores, the goal was to change the structure of the item so that it could become more valuable. -
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's theory was based on the concept that each element consists of its own unique brand of indivisible atom; atoms of one element are all alike but they differ from atoms of other elements. Importantly, Dalton assigned atomic weights to the atoms of the 20 elements he knew of at the time. -
Newton's Law of Octaves
The law of octaves states that when the elements are arranged in ascending order of atomic mass, every eighth element has comparable properties. -
Mendeleev's Pd. Table
Dmitri Ivanovich Mendeleev was a Russian chemist and inventor. He is best known for formulating the Periodic Law and creating a version of the periodic table of elements. -
Photoelectric Effect
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. -
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. -
Charge of the Electron
Electrons 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. -
Discovery of the proton
The proton was discovered by Ernest Rutherford. 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 -
Plum Pudding Model
The plum pudding model is one of several historical scientific models of the atom. First proposed by J. J. Thomson, soon after the discovery of the electron, but before the discovery of the atomic nucleus, the model tried to explain two properties of atoms then known that electrons are negatively charged particles and that atoms have no net electric charge. -
Albert Einstein
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. Furthermore, these molecules are always in random, ceaseless motion. -
Planck's 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. -
Ruther's Gold Foil Experiment
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. -
Robert Millikan
His earliest major success was the accurate determination of the charge carried by an electron, using the elegant “falling-drop method”; he also proved that this quantity was a constant for all electrons, thus demonstrating the atomic structure of electricity. -
Bohr's Planetary Model
According to the Bohr model, often referred to as a planetary model, the electrons encircle the nucleus of the atom in specific allowable paths called orbits. When the electron is in one of these orbits, its energy is fixed. -
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. -
Schrodinger Equation
The Schrodinger 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. 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 versa. -
Discovery of the Neutron
James Chadwick announced that the core also contained a new uncharged particle, which he called the neutron. -
Discovery of the electron
J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. Thomson proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged. Rutherford's gold foil experiment showed that the atom is mostly empty space with a tiny, dense, positively-charged nucleus.
