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460 BCE
Democritus
Believed atoms were uniform, solid, hard, incompressible & indestructible. He also believed they moved in infinite numbers through empty space until stopped -
427 BCE
Plato
Introduced the Atomic theory in which ideal geometric forms serve as atoms & broke down into triangles -
384 BCE
Aristotle
Did not believe in the Atomic theory & taught that all substances were made up of Earth, Fire, Water and Air... not Atoms -
The Alchemists
A person known in art of alchemy. Alchemists laid down the foundation of the Modern Day Periodic Table of Elements -
Lavoisier
Lavoisier believed that matter was neither created nor destroyed in chemical reactions, and in his experiments he sought to demonstrate that this belief was not violated. -
John Dalton
Believed that all matter is made up of atoms, which are invisible -
Newland’s Law of Octaves
the generalization made by the English chemist J.A.R. Newlands in 1865 that, if the chemical elements are arranged according to increasing atomic weight, those with similar physical and chemical properties occur after each interval of seven elements. -
Mendeleev's Periodic Table
The periodic table helped him to determine atomic mass and properties of unknown elements. As he discovered new elements and recorded properties of existing elements to chart them on his periodic table, he found that there were three elements yet undiscovered and would be found at some time in the future. -
Photoelectric Effect
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
March 1, 1896: Henri Becquerel Discovers Radioactivity. In one of the most well-known accidental discoveries in the history of physics, on an overcast day in March 1896, 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. -
Planck's Quantum Theory of Light
Different atoms and molecules can emit or absorb energy in quantities only -
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). -
Rutherford'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. -
Charge of the electron
The elementary charge, usually denoted by e is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 e. This elementary charge is a fundamental physical constant. -
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. -
Moseley's 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 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. -
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 (1910), thus demonstrating the atomic structure of electricity. -
Schrodinger's 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's 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.
