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1783 BCE
The Alchemists
The alchemists believed that all metals were formed from two principles, mercury and sulfur. They contributed to the atomic theory by laying down the foundation of the Periodic Table we are familiar with, of the elements. Alchemists invented experimental techniques distillation, for example, and laboratory tools that are still used by chemists today. -
427 BCE
Plato
He discovered ideal geometric forms serve as atoms, according to which atoms broke down mathematically into triangles, such that the form element. He discovered that there are only five solid shapes whose sides are made from regular polygons. He argued that atoms just crashing into other atoms could never produce the beauty and form of the world. -
400 BCE
Democritus
Democritus believed that atoms were uniform, solid, hard, incompressible, indestructible, and that they moved in infinite numbers through empty space until stopped. He theorized that all material bodies are made up of indivisibly small “atoms.” Democritus also proposed that matter, when small enough, reaches a point where it can no longer be divided and becomes atoms, meaning "indivisible." -
332 BCE
Aristotle
Aristotle pioneered the scientific method of analysis. His theory was that matter was fundamentally constructed out of atoms and that his theory was that a mass of incomprehensible size was everywhere. -
Lavoisier
Antoine Lavoisier believed that matter was neither created nor destroyed in chemical reactions. Antoine Lavoisier's work in defining the law of conservation of mass would help to shape atomic theory. He discovered the role that oxygen plays in combustion and silicon in June. -
John Dalton
His theory was that all matter is composed of extremely small particles called atoms, the first complete attempt to describe all matter in terms of atoms and their properties. Dalton discovered that oxygen combined with either one or two volumes of nitric oxide in closed vessels over water. -
Newland's Law of Octaves
J.A.R. Newlands discovered that if elements are arranged by atomic weight that the ones with similar properties occur after each interval of seven elements. This was the first logical attempt to classify elements based on atomic weights. -
Mendeleev's Periodic Table
His full name is Dmitri Mendeleev's and he created the table because he wanted to find a way to organize the 63 known elements so it would be easier for students to learn about them. The periodic table is based on the atomic mass. He said “Element properties are a periodic function of their atomic weight” when explaining the meaning of the table. -
Photoelectric Effect
Discovered by Heinrich Rudolf Hertz in the late 1800s. It demonstrated that light has particle-like qualities and that we could consider light as photons (packets) of energy where one photon interacts w/ one electron. Each photon must have sufficient energy to remove each electron. -
Discovery of Radioactivity
Henri Becquerel (physics) accidentally discovered radioactivity in March of the late 1800s. He discovered that uranium salts spontaneously emit a penetrating radiation that can be registered by a photographic plate. -
Discovery of the Electron
Joseph John Thomson discovered the electron. He discovered this by deflecting the cathode ray towards a positively charged plate to deduce that the particles in the beam were negatively charged -
Planck's Quantum Theory of Light
Albert Einstein discovered this theory and in Plank's Quantum theory, different atoms and molecules can emit or absorb energy in discrete quantities only. The smallest amount of energy that can be emitted or absorbed in the form of electromagnetic radiation is known as quantum. It explains the quantum nature of the energy of electromagnetic waves. -
Plum Pudding Model
Invented by JJ Thomas, it depicts the electrons as negatively charged particles embedded in a sea of positive charge. This was an important step in the development of atomic theory. -
Discovery of the Proton
The proton was discovered by Ernest Rutherford, he discovered protons from a nuclear reaction that split an atom. -
Rutherford's Gold Foil Experiment
Led by Ernest Rutherford, this was a very important experiment because it showed that the atom is mostly empty space with a tiny, dense, positively charged nucleus. He concluded that a tiny, dense nucleus was causing the deflections. -
Bohr's Planetary Model
Also known as a planetary model, Niels Bohr's theory is that electrons move in fixed shells or orbits. While in orbit each shell has a fixed energy. -
Robert Millikan
he discovered elementary charge and preformed an oil drop experiment. His accomplishments were the design and fine-tuning of experiments which unambiguously confirmed the most important scientific theories of his time, providing the implications for atomic theory. Millikan's work demonstrated that electrons did have a discrete, quantifiable charge. -
Moseley's Atomic Numbers
Henry Moseley discovered this by observing and measuring the X-ray spectra of various chemical elements using diffraction in crystals. 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. The significance was a milestone in advancing the knowledge of the atom. -
Charge of Electrons
Robert Millikan discovered using a falling-drop method. An electron is a fundamental physical constant that is used to express the naturally occurring unit of electric charge which is = 1.602 × 10-19 coulomb. -
Schrodinger Equation
Discovered by Erwin Schrodinger, stated that linear partial differential equation that governs the wave function of a quantum-mechanical system, helped them to detect where the electron could be at any given moment. The significance was that electrons had extremely unpredictable behaviors. Describes how the wave function of a quantum mechanical system (hydrogen atoms electron) evolves. -
Heisenberg Uncertainty Principle
Werner Heisenberg stated, “we cannot know both the position and speed of a particle, such as a photon or electron, with perfect accuracy”. The principle was to help prove that electrons are not present in atomic nucleus. -
Discovery of the Neutron
James Chadwick he discovered the neutron in an experiment where he bombarded Beryllium with alpha particles from the natural radioactive decay of Polonium. This discovery gave scientists a new tool for probing the properties of atomic nuclei.
