Atomic Model Timeline

by maxwell newsom

John Dalton 1803

-Dalton developed the theory that would explain why

the elements in a compound always join the same way.

-Dalton proposed that all matter is made up of individual

particles named atoms,which can not be divided.

#1-All elements are made of atoms

#2-All atoms of the same element have the same

mass and ones of different elements have different


#3-Compounds contain more than one element.

#4-Atoms of different elements always

combine the same way.

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J.J Thomson 1897

-Used electric currents to learn more about atoms

-He used a tube with charged metal disk at each

end and a glowing beam in the middle.

-He hypothesized that the beam in the middle was

a charge stream of particles that interacted with

the air to glow.

-His Experiment provided the first evidence of subatomic particles.

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Hantaro Nagaoka

Hantaro Nagaoka (1865–1950) was a famous Japanese physicist who contributed importantly to the atomic model. Nagaoka was born in Japan and had an education at Tokyo University. He travelled to many places in the world to learn and discover new things about science, meeting many famous scientists on the way. He was also a professor in physics at Tokyo University for a period of time. Hantaro Nagaoka reviewed J.J Thomson's atomic model and disagreed with it, saying that electrons couldn't be located in the positively charged atom. He modified Thomson's model, stating that the negatively charged electrons were actually located outside the atom, and that they orbit around it. His model resembled a mini-solar system.

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Ernest Rutherford

-Ernest Rutherford discov- ered that uranium emits fast-moving particles that have a positive charge. He named them alpha particles.

- In 1909, Rutherford asked one of his students, Ernest Marsden, to find out what happens to alpha particles when they pass through a thin sheet of gold.

-He aimed a narrow beam of alpha particles at the gold. The screen around the gold was made of a material that produced a flash oflight when struck by a fast-moving alpha particle. Byobserv- ing the flash, Marsden could figure out the path of an alpha particle after it passed through the gold.

-More particles were deflected than he expected.

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Niels Bohr

-You may have seen diagrams of an atom that look like a solar system with planets revolving around a sun. These diagrams are based on a model of the atom that was developed by Niels Bohr (1885-1962), a Danish physicist who worked for a while with Rutherford.

-Bohr's model did something that Rutherford's model did not do. It focused on the electrons. A description of the arrangement of electrons in an atom is the centerpiece of the modern atomic model.

-In Bohr's model, electrons move with constant speed in fixed orbits around the nucleus, like planets around a sun. Each electron in an atom has a specific amount of energy. If an atom gains or loses energy, the energy of an electron can change.

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Louis de Broglie 1924

-His ideas were a basis for developing the wave mechanics theory. This theory has greatly improved our knowledge of the physical nature on the atomic scale. He received the Nobel Prize for Physics on his wave nature of electrons discovery in 1929.

-He believed that electrons can act like both particles and waves, just like light. He also said that waves produced by electrons contained in the orbit around the nucleus, set up a standing wave of a certain energy, frequency, and wavelength. He discovered that electrons can act like waves which helped explain some of the things electrons do that we had never been able to explain before. Today, quantum physics is applied in lasers, computers, and microscopes.

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Erwin Shrodinger 1926

In 1913, Neils Bohr, a student of Rutherford's, developed a new model of the atom. He proposed that electrons are arranged in concentric circular orbits around the nucleus. This model is patterned on the solar system and is known as the planetary model. The Bohr model can be summarized by the following four principles:

-1 Electrons occupy only certain orbits around the nucleus. Those orbits are stable and are called "stationary" orbits.

-2 Each orbit has an energy associated with it. The orbit nearest the nucleus has an energy of E1, the next orbit E2, etc.

-3 Energy is absorbed when an electron jumps from a lower orbit to a higher one and energy is emitted when an electron falls from a higher orbit to a lower orbit.

-4 The energy and frequency of light emitted or absorbed can be calculated by using the difference between the two orbital energies.

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James Chadwick 1932

James Chadwick discovered the neutron using evidence collected by Irene Joliot-Curie, who discovered that when beryllium was bombarded with positively charged alpha particles a beam with a high penetrating power was created. James Chadwick discoverd that this beam was not deflected by either electric or magnetic fields, meaning it contained neutral particles- neutrons. Neutrons were found to have the same mass as protons which accounted for more of the mass of the atom and allowed the masses (the known mass of an atom and the known mass of its particles) to match. The common understanding of an atom was now a nucleus containing positively charged protons and neutral neutrons (making up nearly all of the atom's mass) with the rest of the atom (most of it- considering the relative size of the nucleus, which was found to have a raduis of10000 times less than the atom itself) being space in which negatively charged electrons (with a mass so small it is insignificant compared to that of the nucleus) "orbit" the nucleus on energy levels corresponding to the amount of energy the electrons hold.

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