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492 BCE
Deomcritus
Democritus was an Ancient Greek Philosopher that introduced the idea of an atom.He said that atoms are eternal indivisible and indestructible. Also that atoms occupy space and differ from each other in various properties. This theory couldn't explain how new compounds were formed during chemical reactions -
John Dalton
John Dalton was a British physicist,who built upon Democritus theory of atoms. He said atoms are spherical and in motion. While they don't get destroyed during a chemical reaction, their positions switch to create a new element. Molecules have different weights and combine in fixed whole number ratio. -
J.J thomspon(British Physicist )
JJ Thompson proposed that electron roam freely in a region of space that was uniformly positive. Therefore atoms had an overall zero charge. An analogy to visualize this is Plum pudding or chocolate chip cookie. HIs experiment was proven wrong by Rutherford Gold foil experiment. IN which he shot alpha particle at a foil of gold sheet. If J.J. Thompson was correct then all the alpha particles should have passed but, some of them returned back -
Ernest Rutherford (British Physicist , student of J.J. Thomson)
Rutherford develops his theory out of the gold foil experiment. Most of the alpha particle shot at a gold foil passed through, and some got deflected at high angles. This meant that atoms have a positively charged nucleus in the center and surrounding is the electrons. This model has one serious flaw as electron moved freely around the nucleus, it would release energy and thus collapse within 16 picosecond, and everything would cease to exist. This instability was corrected in Bhor' s model -
Niels Bhor ( British Physicist, student of Ernest Rutherford)
Neils Bhor was a Danish physicist who developed upon the Rutherford model of the atom. As Rutherford’s atom had problems with stability, he introduced the idea of stable orbits, where electrons can exist without collapsing in the nucleus. Also, electrons can jump to a higher or lower level by absorbing or releasing energy in exact packets known as quanta. This explained the phenomenon of spectral lines. -
James Chadwick
James Chadwick was British physisct , who discovered neutron. HIs discovery of electron caused unrest in scientific community for some time. James Chadwick theory adopted the Bhor model and added neutron in the nucleus with proton. His experiment helped produce elents heavier than Uranium in labs. -
Period: to
Mordern theory
Wave Mechanical model was developed through the 1920'sand 30's. Erwin Schrodinger and Louis Victor de Broglie came up with the basis of this model as Bhor model was insufficient in locating electrons. Using various mathematics equation they developed a model in which there is a probability cloud of electron that surrounds the nucleus. with further discoveries the model changed in various base but the basis remains the same. All modern electronics are a result if quantum model of atom. -
Murray Gell-Mann (American Physicist)
Murray Gell-Mann proposed particles smaller than subatomic particles known as Quarks. Two independent researchers made this discovery together but he gets the credit. Now instead of protons and neutrons in the center there are different types of quarks. This prediction was proven when the large hadron collider smashed atoms and found proof about quarks. These fundamental particles are still true today
