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460
Democritus
What did he do ?
He had a theory that natures basic particles were atoms but it was not supported by experimental evidence.
How ?
He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided?
Then he made a theory:
Matter couldn't be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible and smallest piece of matter. -
Dalton Atomic Theory
What did he do? He proposed the Atomic theory of matter based on his experimental observations. How? To Dalton Atomic Theory :
1-•All matter is made of atoms. Atoms are indivisible and indestructible.
2-All atoms of a given element are identical in mass and properties.
3-Compounds are formed by a combination of two or more different kinds of atoms.
4-A chemical reaction is a rearrangement of atoms.
5-•Atoms can be neither created nor destroyed. -
Thomson's Atomic Theory
What did he do? Thomson's atomic theory proposed a model of atom which is known as plum pudding model. How?
End of the nineteenth century the concept of atom was similar to a small solid billiard ball. J.J.. Thomson changed the view of an atom by discovering electron (Cathode Rays Experiment) Thomson’s atomic theory suggested that : 1- The atom is not indivisible as it was of smaller pieces – electrons and protons.
2- The electrons were a basic part of all atom. -
Rutherford Atomic Theory
What did he do? He therized that atoms have their protons in a very small nucleus. How ? Ernest Rutherford presented a paper to the Manchester Literary and Philosophical Society accurately describing the structure of the atom. Based on an experiment he had performed (with totally unexpected results) he realized that the atom must have almost all of its mass concentrated at its center, in a nucleus, with the vast majority of the atom consisting chiefly of empty space. -
Bohr Atomic Theory
What did he do? Niels Bohr refined Rutherford's model in 1913 How ? For his theory : 1- The electron in a hydrogen atom travels around the nucleus in a circular orbit.
2-Electrons with low energy would orbit closer to the nucleus while electrons with high energy orbit further from the nucleus.
3- Only limited number of orbits with certain energies are allowed.
4 - Light is absorbed when an electron jumps to a higher energy orbit and emitted when an eectron falls into a lower energy orbit. -
Quantum physical models of the atom
What did Louis de Broglie do?
In 1924, a French physicist named Louis de Broglie suggested that, like light, electrons could act as both particles and waves. How ?
Although the Bohr model adequately explained how atomic spectra worked, there were several problems that bothered physicists and chemists:
• Why should electrons be confined to only specified energy levels?
waves produced by an electron confined in its orbit about the nucleus sets up a standing wave of specific wavelength, energy.
