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400
Aristotle
Around the 5th century (BCE), Aristotle came up with a theory about the atom that differed from Democritus'. Aristotle believed that there were four elements: water, earth, fire, and air. The fifth element was aither, which made up the heavens. The four elements found on Earth contain the traits of being cold, hot, dry, or moist. These element were balanced by the 2 forces of conflict and harmony.
**This theory was founded in BCE, but the timeline would not let me set the date in BCE. -
400
Democritus
Democritus also proposed a theory in the 5th century about the atom. Thought that all the things around us are made from indestructable units called atoms and the void they move around in, Aristotle thought atoms could contunally be divided. Democritus' atom was round and hard unlike Aristotle's, but was not adopted by many others because Aristotle was very influential.
**BCE -
Antoine Lavoisier
Discovered oxygen (1778), nitrogen, hydrogen (1783 ), created the Law of Conservation of Mass, and realized that water was made of oxygen & hydrogen. Dalton based his Atomic Theory on Lavoisier's experiments. He did not change the atomic model, but changed the views that matter could be destroyed or created. -
John Dalton
Created the Modern Atomic Theory in 1800: all matter is formed from atoms, atoms of a given element are the same in size mass and other properties, atoms cannot be divided, atoms of different elements combine in ratios to form compounds, and atoms are combined, rearranged, or separated in chemical reactions. Some is untrue, but helped predict chemical reactions. Atom was like Democritus thought, hard & round, but added more characteristics to it. -
Henri Beqcuerel
Accidentally discovered radioactivity while experimenting with phosphorescence in 1896. Henri did not create a new model of the atom, but changed how people viewed it because radioactive atoms release 3 different rays. -
J.J. Thomson
Found the electron using a Crooke's tube in 1897. He then proposed that the atom was not a hard sphere that was unable to be didvided, but was negatively charged electrons floating in postively charged matter, like plum pudding, chocolate chip cookie dough, etc. Found another part of the atom, when it was first thought to be hard and indivisble. -
Max Planck
Planck created the Quantum Theory idea in 1900, where energy is released in equal packets,and also created Plnack's constant, used to find to find the energy of light. Planck's contributions didn't improve the atomic model, but helped explain why excited elements glow. -
Marie & Pierre Curie
Shared the Nobel Peace Prize with Henri Becquerel in 1903, because they researched more on Henri’s discovery of radioactivity. They investigated uranium pitchblende, which had a higher amount of radioactivity than pure uranium. Marie then concluded that the pitchblende had an undiscovered element that had a higher radioactivity than uranium. Refining several tons of pitchblende, and found two new elements: polonium & radium. Discovery does not change the model of the atom, but found 2 elements -
Ernest Rutherford
In 1909, he shot alpha particles at a thin sheet of gold, which led him to believe that the atom was mainly empty space, with a dense positivley charged core. Discovered the proton, which allowed him to create better model than Thomson (his teacher) where the electrons orbited around the nucleus of protons like planets around the sun. -
Robert Millikan
Thomson had predicted the size of an electron, but Millikan was able to prove what the mass and charge of an electron in his oil drop experiment in 1910.This did not improve the atom model, but it proved that the electron charge was quantized, which helped prove the quantum theory created by Planck. -
Henry Mosely
Before, the first Perodic Table was arranged by atomic mass. After exposing elements to x-rays, he found that there was a relationship between the x-ray frequency & the atomic number, so he arranged the Periodic table by atomicnumbers. This fixed some inconsistencies int Peridic Table, and was even able to predict undiscovered elements. Today his idea is used. Mosely did not alter the atomic model. -
Niels Bohr
Bohr's atom model improved upon Rutherford's in 1915. Rutherford's model had issues b/c physics said particles would loose energy, and become attracted to the nucelus. Bohr's model still had the dense nucleus in the middle, but the electrons follwed specific energy paths according to their energy levels. Now the electrons could stay in the same path. -
Erwin Schrodinger
Schrodinger created the gave an equation to find the location and the energy of an hydrogen atom as a standing wave. This This led to the quantum mechanical model also restricted the electron energies into quantized energies, but unlike the Bohr model the electrons didn't have a fixed path. The model has the nucleus in the center, but the electrons has irregular orbitals. -
Werner Heisenberg
Heisenberg, former student of Bohr, created the Uncertainty Principle, which states that there is no possible way to know exactly know where an electron is on its orbit. This explained why Bohr could not find the frequecies of atomes other than hydrogen. This principle did not improve the atomic mdel, but helped explain Bohr's model. -
James Chadwick
Chadwick, former student of Rutherford, discovered the neutron. Many were satisfied with Rutherford's model, but Chadwick continued his research about the atom. He then found the neutron, the subatomic particle with no charge, in the nucleus. Made cloud model, which was the nucleus surrounded by a region where the electrons could be, like a cloud.
