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Period: 300 to
The ATOMIC Timeline
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500
The Solid, Indivisible Sphere
This is the first idea of an atom. This sphere was created by Democritus. His theory states that atoms are eternal, invisible and so small that they can’t be divided, and they entirely fill up the space they’re in. The Democritus Atomic Theory revolves around the atoms that are present in the atmosphere. These are about the atoms that are present in all the forms of existence; for instance, solid or liquid. The theory states that these atoms are all individually created and cannot be separated, -
500
Democritus Himself
Democritus was a famous personality in Greece because of his many philosophical ideologies. He was regarded with respect and was a powerful figure in his days. His theories about atoms are still studied today, although there have been many people who have claimed that his theory does not hold the actual truth. -
Dalton's Atomic Theory
Dalton's atomic theory remains valid in modern chemical thought. His theory includes:
All matter is composed of atoms
Atoms cannot be made or destroyed
All atoms of the same element are identical
Different elements have different types of atoms
Chemical reactions occur when atoms are rearranged
Compounds are formed from atoms of the constituent elements. -
Dalton's Atomic Model
All matter is composed of atoms
Atoms cannot be made or destroyed
All atoms of the same element are identical
Different elements have different types of atoms
Chemical reactions occur when atoms are rearranged
Compounds are formed from atoms of the constituent elements. -
John Joseph Thomson
In 1897 Sir John Joseph Thomson discovered that electrons can be stripped off of metal atoms. He did this by using a glass tube that had most of the air removed. In the tube were two metal plates. One had a high negative charge and the other a high positive charge. -
Plum Pudding Model
Thomson proposed that the atom had these negatively charged corpuscles (later called electrons) that floated in a cloud of positive charge. So the electrons were like plums baked in a bowl of pudding. The positive charges (protons) filled the whole atom (pudding). -
Cathode Ray Tube
Thomson had an inkling that the ‘rays’ emitted from the electron gun were inseparable from the latent charge, and decided to try and prove this by using a magnetic field.
His first experiment was to build a cathode ray tube with a metal cylinder on the end. This cylinder had two slits in it, leading to electrometers, which could measure small electric charges. -
The Gold Foil Experiment
A famous experiment by Ernest Rutherford, revealed something new about a proton. A radioactive source emitted alpha particles that shot out in one direction towards a thin sheet of gold foil. The particles appeared to pass right threw the gold foil as if is wasn't there. Flashes of light would appear on the fluorescent screen as the alpha particles passed through the foil and slammed into it. Every once in awhile an alpha particle would hit the screen off to the side bounced backwards. -
Rutherford's Atomic Model
used the results of his gold-foil experiment to state that all the mass of an atom were in a small positively-charged ball at the center of the atom. -
Rutherford's Atomic Theory
Ernest Rutherford publishes his atomic theory describing the atom as having a central positive nucleus surrounded by negative orbiting electrons. This model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space. Rutherford came to this conclusion following the results of his famous gold foil experiment. -
Neil Bohr's Atomic Theory
Niels Bohr applies quantum theory to Rutherford's atomic structure by assuming that electrons travel in stationary orbits defined by their angular momentum. This led to the calculation of possible energy levels for these orbits and the postulation that the emission of light occurs when an electron moves into a lower energy orbit. -
Bohr's Atomic Model
Electrons revolve around the nucleus in certain special orbits and each of these orbits has different energy levels associated known as energy shells (or energy level measured in electron volt). The orbit closest to the nucleus has the lowest energy level and the orbit farthest to the nucleus has the highest energy level. Since electrons move in a particular orbit, they do not lose energy. When an atom absorbs energy, these electrons get excited and jump to the next higher energy level. -
Arthur H. Compton
In 1922, the American physicist Arthur H. Compton (1892–1962) conduced x-ray scattering experiments that confirmed and advanced Einstein's theory on the dual nature of light. -
Erwin Schrödinger's Atomic Theory
Erwin Schrödinger built upon the thoughts of Bohr yet took them in a new direction. He developed the probability function for the Hydrogen atom (and a few others). The probability function basically describes a cloud-like region where the electron is likely to be found. It can not say with any certainty, where the electron actually is at any point in time, yet can describe where it ought to be. The model based on this probability equation can best as a cloud model. -
Schrödinger's Atomic Model
The cloud model represents a sort of history of where the electron has probably been and where it is likely to be going. The red dot in the middle represents the nucleus while the red dot around the outside represents an instance of the electron. Imagine, as the electron moves it leaves a trace of where it was.This collection of traces quickly begins to resemble a cloud. -
James Chadwick and Neutrons
James Chadwick proved the existence of neutrons -
Nuclear Bombing: The Explosions of Hiroshima and Nagasaki
The first nuclear bomb meant to kill humans exploded over Hiroshima, Japan, on Aug. 6, 1945. Three days later, a second bomb detonated over Nagasaki. The death and destruction wrought by these weapons was unprecedented and might have, in another world with another race of beings, ended the nuclear threat right then and there. -
Radiocarbon Dating
A radiometric method for measuring the decay of the radioactive isotope carbon- 14 in organic material up to 80 000 years old, developed in 1948-9 by Willard Libby. -
The Modern Day Atomic Model and Quantum Theory
The quantum mechanical model is based on quantum theory, which says matter also has properties associated with waves. According to quantum theory, it’s impossible to know the exact position and momentum of an electron at the same time. This is known as the Uncertainty Principle.
The quantum mechanical model of the atom uses complex shapes of orbitals (sometimes called electron clouds), volumes of space in which there is likely to be an electron. -
Aristotle Atomic Theory
Aristotle’s theory made a great generalization off all matter being made of the four elements: fire, water, earth, and air. He also believed that there were four qualities to these elements: dryness, hotness, coldness, and moistness. Based on these beliefs fire would hold the characteristics of being dry and hot, water is wet and cold, air is hot and wet, while the earth is dry and cold.
