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450 BCE
Plato
Plato believed more in abstract ideas rather than the physical world so he valued the idea that geometric forms served as atoms based on which atoms broke down into triangles, mathematically. Fire was tetrahedron, air was octahedron, water was icosahedron and earth was cube. -
430 BCE
Democritus
Democritus believed that atoms changed shape based on what they composed. He named the building blocks of matter "atomos" which means indivisible. He also believed that they were in constant motion and collided with each other among, other things. Plato and Aristotle attacked his atomic theory because they believed in more philosophical ideas than physical ones. -
332 BCE
Aristotle
Aristotle did not believe in atoms, he believed that all matter was made up of the 4 elements, fire, water, air and earth. -
Period: 700 to 1500
The Alchemists
Alchemists contributed to the atom by breaking down the chemical composition of the base elements of the period, fire, earth, wind and water which led to the laying of the foundation to modern the periodic table. -
Lavoisier
Antoine Lavoisier was considered to be the founder of modern chemistry. He was able to prove the law of conservation of mass. -
John Dalton
John Dalton's significant contribution to the atom was that he created the theory that only like atoms in a mixture of gases would repel one another where atoms that were different would attract one another. -
Newland's Law of Octaves
J.A.R. Newland's found that if you arrange the chemical elements by weight, the elements with similar physical and chemical properties occur after each interval of seven elements. -
Mendeleev's Pd. Table
Mendeleev's periodic table sorts the known elements of the time by their atomic weight. Dmitri Mendeleev discovered periodic law which states atomic weights of elements could be used to arrange the elements into groups but also to arrange the groups themselves. -
Photoelectric Effect
The photoelectric effect is a phenomenon in which particles that are electrically charged are released from or within a material when it has absorbed electromagnetic radiation. The photoelectric effect was discovered by Heinrich Rudolf Hertz. -
Discovery of Radioactivity
Henri Becquerel accidently discovered radioactivity when he was doing an experiment to see if uranium salts would absorbs sunlight and then re-emit it as x-rays. He did the experiment and got results and wanted to keep testing but the weather became overcast. After several days he took the plate out and developed it and accidently found that the image was extremely clear. -
Discovery of the electron
Sir Joseph John Thomson discovered the electron when he decided to replicate an experiment from Heinrich Hertz. While doing the experiment he discovered that cathode rays were negatively charged particles which allowed him to determine the nature of electricity and thus calling the particles electrons. -
Planck's Quantum Theory of Light
Planck's quantum theory states that different atoms and molecules can only emit and absorb energy in individual quantities. This theory was determined by Max Planck. -
Plum Pudding Model
The plum pudding model, or sometimes known as Thomson atomic model, was created by Sir Joseph John Thomson in which he described the atom as a uniform sphere of positively charged matter in which electrons are embedded. -
Charge of the Electron
Robert Millikan discovered the charge of the electron when doing his famous oil drop experiment. -
Robert Millikan
Robert Millikan is known for his famous oil drop experiment which was used to determine the charge carried by a single electron. He was able to find the charge by dripping a drop of oil between two metal plates and sending varying levels of electric voltage until the drop of oil stopped falling. -
Rutherford's Gold Foil Experiment
The Rutherford gold foil experiment was performed by Ernest Marsden under the direction of Ernest Rutherford. The experiment was done by using a focused beam of radiation and shooting at a thin piece of gold foil. The experiment was trying to determine if the atoms would go through the gold foil or if they would be reflected. Since most of the particles went through the gold foil, it was then determined that atoms were mostly open space. -
Bohr's Planetary Model
Niels Bohr created a model, known as a planetary model, to discover that electrons circle the nucleus of an atom in specific paths known as orbits. -
Moseley's Atomic numbers
Henry Mosley determined that the atomic number is the number of positive charges in the atomic nucleus. He also stated that there were three elements with the atomic numbers 43, 61 and 75 between aluminum and gold on the periodic table. -
Discovery of the proton
Ernest Rutherford discovered the proton when during his research a nuclear reaction occurred causing the first splitting of the atom. -
Schrodinger's Equation
Schrodinger's equation is the fundamental equation for quantum mechanics. It describes the form of the probability of waves that control small particles and specifies how the waves are affected by external sources. It was developed by Erwin Schrodinger. -
Heisenberg's Uncertainty Principle
Heisenberg's uncertainty principle, determined by Werner Heisenberg, states that the position and velocity of an object can't be measured exactly, at the same time even theoretically due to all the uncertainties implied. -
Discovery of the Neutron
James Chadwick determined that the nucleus of an atom also contained an uncharged particle, which he named the neutron. He discovered the neutron while doing an experiment that included firing alpha radiation at a sheet of beryllium from a polonium source. He observed penetration of uncharged particles which he determined to be neutrons.