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4000 BCE
Democritus
Democritus believed that everything in the universe was made up of atoms, which were microscopic and indestructible.He took a seashell and broke it in half. He then took that half and broke it in half over and over and over and over again until he was finally left with a fine powder. He then took the smallest piece from the powder and tried to break that but could not.According to Democritus atoms were miniscule quantities of matter. -
John Dalton
Dalton assumed that water contains one atom of hydrogen and one atom of oxygen and concluded that the relative weight of the oxygen atom must be 5.6 times as large as the hydrogen atom. -
Sir J.J. Thomson Part 1
The first is the experiment of Joseph John Thomson, who first demonstrated that atoms are actually composed of aggregates of charged particles. Prior to his work, it was believed that atoms were the fundamental building blocks of matter. -
Sir J.J Thomson Part 2
The first evidence contrary to this notion came when people began studying the properties of atoms in large electric fields.If a gas sample is introduced into the region between two charged plates, a current flow can be observed, suggesting that the atoms have been broken down into charged constituents. The source of these charged particles is a heated cathode that, in fact, causes the atoms of the sample to ionize. -
Lord Rutherford Part 1
A consummate experimentalist, Rutherford discovered alpha and beta rays, set forth the laws of radioactive decay, and identified alpha particles as helium nuclei. Most important, he postulated the nuclear structure of the atom: experiments done in Rutherford’s laboratory showed that when alpha particles are fired into gas atoms, a few are violently deflected, which implies a dense, positively charged central region containing most of the atomic mass. -
Lord Rutherford Part 2
The Rutherford model is a model of the atom devised by Ernest Rutherford. Rutherford directed the famous Geiger–Marsden experiment in 1909 which suggested, upon Rutherford's 1911 analysis. -
Niels Bohr Part 1
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. Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted. -
Niels Bohr Part 2
In atomic physics, the Rutherford–Bohr model or Bohr model or Bohr diagram, introduced by Niels Bohr and Ernest Rutherford in 1913, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus -
Sir James Chadwick Part 1
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. -
Sir James Chadwick Part 2
Thus, this model introduced the concept of sub-energy levels. 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. -
Current Model
Schrodinger and Heisenberg Model. There is a key point about the Bohr model that is no longer accepted in current models of the atom. In the Bohr model, the electrons are still thought to orbit the nucleus just like planets orbit the sun. Actually, this is something that we can not say is true.