Atomic Models

The atomic model by John Dalton

John Dalton's most important theory, developed in 1803, was that matter is composed of atoms of differing weights that combine in simple ratios.

Dalton published a book in 1808 listing the atomic weights of a number of known elements relative to the weight of hydrogen. Although not entirely accurate when published, those weights form the basis for the modern periodic table of the elements.

Dalton Atomic Theory discusses the nature of atoms. Although a product of the research of 19th century chemist John Dalton, it remains valid and important today.

There are four essential parts of 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.

All atoms of the same element are identical

Different elements have different types of atoms

Chemical reactions occur when atoms are rearranged

Compounds are formed from atoms of the constituent elements.

Dalton's theory was based on the premise that the atoms of different elements could be distinguished by differences in their weights. He stated his theory in a lecture to the Royal Institution in 1803.

Using his theory, Dalton rationalised the various laws of chemical combination which were in existence at that time. However, he made a mistake in assuming that the simplest compound of two elements must be binary, formed from atoms of each element in a 1:1 ratio, and his system of atomic weights was not very accurate - he gave oxygen an atomic weight of seven instead of eight.

Despite these errors, Dalton's theory provided a logical explanation of concepts, and led the way into new fields of experimentation.

The atomic model by J.J. Thomson

Thomson discovered the electron in the year 1897. His work put forward a new theory, that atom was made up of small particles. Thus he discovered the electrons. He proved his theory using the cathode ray tube.

Scientists had already done many experiments to find the structure of the atom. They passed an electric current through a vacuum tube. They saw a light glowing inside the tube. It travelled in a straight line. They could not explain the phenomenon. Thomson did more experiments on the cathode tube. He placed two electric plates on its path. One was positively charged. The other was negatively charged. The glowing light bent towards the positively charged plate. And found that the glowing materials bent towards the positively charged plate. He found out that the glowing light were smaller particles. They were smaller

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