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460 BCE
Democritus
The Ancient Greek Philosopher creates the classical atomic theory in which the universe is made of atoms that are indestructible that vary in size and shape -
Period: 3 BCE to
Alchemists
Practicians of an ancient branch of natural philosophy called alchemy. They believed the world is composed of 4 basic elements of air, earth, water, and fire, with three essential substances of salt, sulfur, and mercury. Sought to turn lead into gold in the belief that lead is an immature metal, and once converted to gold, will become perfect with a harmonious balance between the 4 basic elements. -
Antoine-Laurent Lavoisier & the Law of Conservation of Mass
Antoine-Laurent Lavoisier, known as the "Founding Father of Chemistry," discovers that the law of conservation of mass is a fundamental principle of physics and states that mass is conserved and cannot be created or destroyed within an isolated system. This shows that the mass of the product is always equal to the mass of the reactants. -
Joseph-Louis Proust & The Law of Definite Proportion
Proven by French chemist Joseph-Louis Proust, who found that relative quantities of any chemical compound’s elements contained the same ratio by their weight, no matter the source, states that every chemical compound contains fixed and constant proportions (by mass) of its constituent elements. -
John Dalton & Dalton's Atomic Theory
Dalton was an English chemist who laid the foundation for the modern atomic theory, and in the basis of Democritus' theory of the atom's existence, he proposed his atomic theory in that all matter is made of atoms, which are indivisible and indestructible, and that atoms of the same element have identical mass & properties, compounds are combinations of atoms of 2 or more elements, and chemical reactions are the rearrangements of atoms. -
Dmitri Mendeleev
A Russian chemist, Mendeleev organized matter in order of increasing weight, observed patterns within the properties within the group of elements, which then led to the development of the very first version of the periodic table. -
J.J. Thomson & The Cathode Ray Tube
Thomson, a British physicist, conducted experiments with the Cathode Ray Tube and came to the discovery of negatively-charged particles, come to be known as electrons, the fundamental building block that can be found in atoms of all elements. -
The Plum Pudding Model
With the prior experiments conducted by Thomson to the CRT, Thomson proposes an atomic model built up of negatively-charged electrons within the atom that was very small compared to the entire positively-charged substance, and that the electrons are scattered within the substance. -
Period: to
The Gold Foil Experiment
An experiment conducted by Ernest Marsden under the direction of Ernest Rutherford and German physicist Hans Geiger, alpha particles were directed at a gold foil, with most of the particles passing through the foil, but some of the alpha particles being deflected back, which disproved Thomson’s proposed atomic model. this also showed that gold atoms were mostly made up of empty space. -
Robert Millikan & The Oil Drop Experiment
American physicist and Nobel Peace prize winner Robert Millikan conducted the Oil Drop experiment alongside Harvey Fletcher, in which they observed electrically charged oil droplets sprayed in a sample chamber with two plates that had a + charge on top with a pinhole and - charge underneath to which an x-ray was used to ionize the droplets in order to measure the electrical charge of an electron. They confirmed Thomson's hypotheses of an electron being 1000 times smaller than the lightest atom. -
Ernest Rutherford & The Nuclear Model
Ernest Rutherford, a British physicist and a Nobel Prize laureate in Chemistry, proposed the nuclear atomic model, also known as the Rutherford Atomic Model, after the conclusions that were made in his gold foil experiment conducted by him and his associates which disproved Thomson's Plum Pudding Model. His nuclear atomic model was built where negatively-charged electrons orbit around the positively-charged protons in the nucleus which had most of its mass concentrated in it. -
Niels Bohr & The Planetary Model
Danish physicist and Nobel Prize laureate in Physics Niels Bohr proposed the Planetary Atomic Model which modified Rutherford's Nuclear Model by fixing the instability issue of Rutherford's randomized electron orbit around the nucleus and proposing that the electrons require to move in fixed orbits of different sizes and energies and that radiation occurs only when electrons jump from orbit to orbit. -
Henry Moseley
An English physicist, Moseley demonstrated that the atomic number had determined the major properties, which established the relationship of atomic number and charge of the nucleus. This was done via the emission of x-rays unto each element, and with that, he was able to find the difference between each element and that they are defined by their proton. With this process, it allowed for the more accurate organization of matter within the periodic table. -
Period: to
Werner Heisenberg
A German theoretical physicist during the first world war, Heisenberg won a Nobel Prize for his creation of the theory of quantum mechanics. In 1926, he also created the uncertainty principle. -
Erwin Schrodinger & The Quantum Mechanical Model
An Austrian physicist, Schrodinger proposed the Quantum Mechanical Model, also known as the Electron Cloud Model, which arose from his mathematical equation's solution, wave functions, which gave the probability of finding an electron in a certain point around the nucleus in the area called the electron cloud. The density of the electron cloud determines the probability of an electron’s location around the nucleus. With the proposal of the model, it introduced the concept of sub-energy levels. -
James Chadwick
An English physicist, Chadwick was the one who discovered a radiation composing of neutral electric charge which contained the approximate mass of a proton, which later came to be known as a neutron. His discoveries were very important to the finding of nuclear fission in later years.