-
460 BCE
Democritus Theory of the Universe
Democritus was a Greek philosopher who was the first person to use the term atom. He thought if you take a piece of atom, you can divide it until you can't anymore. He called the basic unit an atom. He made the Theory of the Universe. -
427 BCE
Plato
Plato introduced the atomic theory in which ideal geometric forms serve as atoms. Atoms broke down into triangles and the form elements had different shapes. Fire (tetrahedron), air (octahedron), water (icosahedron), earth (cube). -
384 BCE
Aristotle
Aristotle's theory was supported by observing everyday life. He believed that all matter was made up of five elements. Earth, fire, air, water, aether. -
The Alchemists
Alchemy started back in the 8th century and lasted till 1720. They had a theory that are metals are made of mercury and sulfur and that it is possible to change base metals into gold. -
Lavoisier
Antione Lavoisier came up with the law of conservation of mass. In a chemical reaction mass cannot be created or destroyed. -
John Dalton
He was the first person to adapt Democritus's theory into the first modern atomic model. All matter consists of atoms, Atoms are indestructible and unchangeable, Elements are characterized by their weight of their atoms, and When elements react their atoms combine and make new compounds. -
Newland's Law of Octaves
Newland arranged the elements by their atomic weights and found out that every eight element has similar properties has the first. -
Dmitri Mendeleev
He is responsible for the periodic table. In the 1860s there were only 60 known elements to put on the table -
Photoelectric effect
Discovered by Heinrich Rudolf Hertz. Photoelectric effect is electrically charged particles are released from a material when it absorbed electromagnetic radiation. -
JJ Thomson
JJ Thomson is credited for discovering the electron. He used his research on cathode ray tube technology in his discovery. He took a tube with the air sucked out and passed electricity through. He then determined the electrons charge was negative. His model was known as the plum pudding model. Each atom is a sphere filled with positively charged fluid resembling the sticky jam. Electrons are negatively charged particles suspended in the fluid representing the plums. -
Discovery of Radioactivity
Henri Becquerel discovered Radioactivity. He was using naturally fluorescent minerals to study the properties of x-rays. He then exposed potassium uranyl sulfate to sunlight and placed it on plates and wrapped them in black paper. -
Max Planck
Planck's quantum theory of light is different atoms and molecules can emit or absorb energy in small quantities only. The smallest amount of energy that can be emitted in the form of electromagnetic radiation is known as quantum. The energy of the radiation absorbed is the same frequency of the radiation. -
Niels Bohr
Agreed with planetary model but also knew it had mistakes. He made a new model. Electrons orbit the nucleus in orbits with a set size and energy. The lower energy of electron the lower the orbit. As electrons fill up rings, they fill lower energy levels first. If all energy levels are full a new one will start. -
Ernest Rutherford
He was not convinced by the model of Thomson. He conducted the gold foil experiment. He fired positively charged alpha charged at a gold foil. The nucleus of the atom is a dense mass of positively charged particles. The electrons orbit the nucleus. Rutherford said that the atom was like a mini solar system and that the electrons orbited the nucleus. He discovered the proton when he saw that his scintillation detectors found hydrogen nuclei. -
Mosley's Atomic Numbers
Henry Moseley looked at the X-ray spectra of elements that occupied consecutive places in the periodic table. These spectra were found by the method of diffraction of X-rays. Moseley discovered a mathematical relationship between the wavelengths of the X-rays produced by the targeted elements and their atomic numbers. -
Erwin Schrodinger
An electron does not follow an exact orbit and we can only predict where it will go. Schrodinger's equation is linear partial differential equation that helps the wave function of a quantum-mechanical system. -
Robert Millikan
In 1910 he found out the magnitude of an electrons charge. He experimented dropping two drops of oil on two electric plates. By showing how the drops move he found that their charge was a multiple of precise charges. -
Heisenberg Uncertainty Principal
This principal was formulated by Werner Heinsberg. The uncertainty principle says that we cannot know the position and speed of a particle with 100% accuracy. The more we know about the particle's position, the less we know about its speed. -
Discovery of Neutron
In 1932, James Chadwick discovered an uncharged neutron. He did an experiment in which he overwhelmed beryllium with alpha particles from the radioactive decay of polonium.