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400 BCE
Democritis
This is Democritus' atomic theory exactly:
1.All matter consists of invisible particles called atoms.
2. Atoms are indestructible.
3. Atoms are solid but invisible.
4. Atoms are homogenous.
5. Atoms differ in size, shape, mass, position, and arrangement.
->Solids are made of small, pointy atoms.
->Liquids are made of large, round atoms.
->Oils are made of very fine, small atoms that can easily slip past each other. -
John Dalton
Dalton's atomic theory was the first complete attempt to describe all matter in terms of atoms and their properties.
Dalton based his theory on the law of conservation of mass and the law of constant composition.
The first part of his theory states that all matter is made of atoms, which are indivisible.
The second part of the theory says all atoms of a given element are identical in mass and properties.
The third part says compounds are combinations of two or more different types of atoms. -
Sir J.J. Thomson
He discovered a method for separating different kinds of atoms and molecules by the use of positive rays, an idea developed by Aston, Dempster and others towards the discovery of many isotopes. -
Lord Rutherford
Rutherford proposed that atoms consisted of a small dense center filled with positive charges. He named this center the nucleus. He then said that negatively charged electrons were scattered surrounding this dense, positively charged center. He stated these negatively charged electrons were held in orbit by the positively charged nucleus, due to the electrical forces. -
Niels Bohr
The discoveries of the electron and radioactivity at the end of the 19th century led to different models for the structure of the atom. In 1913, Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities. When jumping from one orbit to another with lower energy, a light quantum is emitted. Bohr's theory could explain why atoms emitted light in fixed wavelengths. -
Sir James Chadwick
Until 1932, the atom was believed to be composed of a positively charged nucleus surrounded by negatively charged electrons. In 1932, James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. ... This particle became known as the neutron. -
current model
The atom is made of protons, neutrons and electrons
Most of the space is taken up by the area where the electrons exist
The protons and neutrons are in the core of the atom – called the nucleus
