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332 BCE - Aristotle's Discovery
Although the date is approximate, Aristotle concluded in his Metaphysics book that all things are made up of form and matter which can only be changed with growth, decay, or motion. Objects are in turn composed of the five elements he believed existed: fire, water, earth, air, and aether. -
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Democritus
Aprroximate date. One of the creators of the first basis for the Atomic Theory which stated that all objects are made up of little indestructible and indivisible things called atoms. However, his discovery was often questioned because he could not explain the chemical side of his 'atom' idea (since then have been discovered ex. reaction rate, oxidation etc) -
Cell Discovered
Cell first discovered by Englishman, Robert Hooke -
Joseph Proust's Discovery
He created Proust's Law, or the Law of Definite Proportions. By comparing an artificial element with a natural element, Proust proved that any pure compound has the exact same proportion of weights, regardless of where the elements come from (artificial vs natural) : elements have the same weights no matter, -
John Dalton's Discovery
An English chemist, Dalton created a scientific theory based off of and including Democritus' atom idea: All elements are created from indestructible particles called atoms which are different from other element's atoms. Atoms are the same in same elements though. His theory also introduced compounds, which are made when different elements combine. The final part of his theory was that when atoms of different elements are joined, separated, or rearranged chem -
First Periodic Table Created
1 Jan 1869 Introduced by Russian chemist Dmitri Ivanovich Mendeleev. -
Henri Becquerel's Discovery
While studying phosphorescence of uranium salts, Becquerel noticed that the photographic plates had already been exposed despite not being exposed to bright sunlight. He realized it was due to natural radioactivity which was then referred to as 'spontaneous radioactivity'. -
J. J. Thompson's Discovery
An English physicist who invented the Cathode Ray. When high energy was moved from the negative cathode plate through a vacuum sealed container to a positive plate, it created a beam of electrons. This resulted in the Thompson/Plum Pudding atomic model. In the model, Thompson thought all of the particles he saw were clumped together (much like plum pudding = nickname) This allowed solids to be made when harder substances were clumped together. -
Max Planck's Discovery
1 Jan 1900 Radiation experimentation with black body (object that absorbs all electromagnetic radiation) led to his creation of the Planck Postulate that described the stability of electrons and other subatomic particles. E=hv. With this equation, particle properties of waves were able to be used to describe particles' radiation and movement as well, resulting in the debate of whether objects existed as particles or wavelengths. -
Lord Earnest Rutherford Discovery
1 Jan 1910 While conducting his gold foil alpha particle scattering experiment, Rutherford observed a change in what should have been seen. Instead of the alpha particles going straight through the foil with little to no deflections, the beam deflected in multiple directions leading him to determine that atoms had a nucleus. This led to the making of the Rutherford atomic model a year later in 1911 -
Robert Millikan's Discovery
Using the falling-drop method which balanced downward gravitational pull with buoyant and electrical upward forces, between two electrodes, Millikan found the charge of an electron to be slightly negative. It's approximately described as 1/1840 the mass of a Hydrogen atom. This discovery would later allow scientists to better understand and find the weight of other subatomic particles -
J.J. Thompson's Discovery #2
Thompson discovered isotopes as a result of sending ionized Neon through a magnetic and electric field. The mirrored image produced showed two different parabolas meaning there were tow different atomic masses. -
Niehls Bohr Discovery
Using Planck's Quantum Theory (Planck Postulate) and Rutherford's study of the nucleus in atoms, Bohr came to the conclusion that atoms have a heavily charged nucleus with the electrons at a distance circling around it. This would later turn out to be the first accurate model of the atom. -
Louis de Brogile's Discovery
A French physicist, Louis studied the stability of the atom, using Max Planck's studies as a starting point. After studying both forms of energy that were particle and ones that were wavelengths, he determined that all forms of energy exist in both particles and wavelengths, establishing the wave-particle duality of nature. This ended the debate over particle vs wavelength that started with Planck. -
Erwin Schrodinger's Discovery
Based off of Louis de Brogile's work, he came up with the quantum mechanical atomic model which shows the orbitals around the nucleus in constant motion, equidistant from each other and at different angles. This was made to describe the functions of waves. -
Werner Heisenberg's Discovery
He realized that there are many things about atoms which cannot be exactly determined or calculated. This is mainly directed at determining an electrons location and velocity at the same time. He found it impossible to track or predict where an electron will be at any given moment. This also means that it's impossible to determine for the other subatomic particles as well. This is called the Uncertainty Principle. -
James Chadwick's Discovery
He discovered the existence of neutrons and that they had no charge at all. This meant that there was no electrical barrier to go through when trying to achieve atomic disintegration. This subsequently changed how the model of an atom would look like, and also allowed for atomic bombs to be made because of the presence of neutrons.