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In 1926 Erwin Schrödinger, an Austrian physicist, took the Bohr atom model one step further. Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom.

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Dalton's Atomic Theory (1804)

From his own experiments and observations, as well as the work of his peers, Dalton proposed a new theory of the atom. This later became known as Dalton's atomic theory. The general tenets of this theory are as follows:

All matter is composed of extremely small particles called atoms.

Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ in size, mass, and other properties.

Atoms cannot be subdivided, created, or destroyed.

Atoms of different elements can combine in simple whole number ratios to form chemical compounds.

In chemical reactions, atoms are combined, separated, or rearranged.

Dalton's atomic theory has been largely accepted by the scientific community, with the exception of three changes. We now know that (1) an atom can be further subdivided, (2) all atoms of an element are not identical in mass, and (3) using nuclear fission and fusion techniques, we can create or destroy atoms by changing them into other atoms.

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J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. Thomson proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged "soup."

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Bohr model, description of the structure of atoms, especially that of hydrogen, proposed (1913) by the Danish physicist Niels Bohr. The Bohr model of the atom, a radical departure from earlier, classical descriptions, was the first that incorporated quantum theory and was the predecessor of wholly quantum-mechanical models. The Bohr model and all of its successors describe the properties of atomic electrons in terms of a set of allowed (possible) values. Atoms absorb or emit radiation only when the electrons abruptly jump between allowed, or stationary, states. Direct experimental evidence for the existence of such discrete states was obtained (1914) by the German-born physicists James Franck and Gustav Hertz.

Immediately before 1913, an atom was thought of as consisting of a tiny positively charged heavy core, called a nucleus, surrounded by light, planetary negative electrons revolving in circular orbits of arbitrary radii.

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Rutherford atomic model
In the early part of the 20th Century physicist Ernest Rutherford, and his team, carried out a series of experiments exploring the atom by shooting high speed positively charged alpha particles at a thin piece of gold foil. Alpha particles were known from earlier work that had been done in radioactivity, but scientists were still not sure if they were bits of atoms. Alpha particles were later discovered to be the nuclei of helium atoms.

What Rutherford discovered was that most of the alpha particle "bullets" passed straight through the gold foil, as though it were empty space. The occasional particle was diverted from its course by something very small and dense in the gold atoms.

The Rutherford atomic model was also known as the "Rutherford nuclear atom" and the "Rutherford Planetary Model". In 1911, Rutherford described the atom as having a tiny, dense, and positively charged core called the nucleus. Rutherford established that the mass of the atom is concentrated in its nucleus. The light, negatively charged, electrons circulated around this nucleus, much like planets revolving around the Sun.

Rutherford's model varied from Dalton's and Thompson's models in that he considered that most of the atom was empty space, whereas in the earlier models the atom was considered solid.

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Dalton's Atomic Theory

Explanation of :

- Law of conservation of mass
- Law of definite proportions
- Law of multiple proportions

- Video!