# Atomic structure|What is Atomic structure ?|Definition

Atomic structure : The modern theory of the atomic structure of matter was first propounded by the British chemist, John Dalton in 1803 on the basis of a number of well-known laws of chemical combination. These include the law of conservation of mass, the law of definite proportions and the law multiple proportions.

## ATOMIC STRUCTURE OF MATTER

Dalton’s atomic theory It is well-known to the students of modern science that all matter is made up of microscopically small particles known as atoms, and the theory is called Atomic Structure theory.

This atomic view of matter was known to the ancient philosophers in different parts of the world, including Democritus (400 B.C) in Greece and Canada in India. However, the ancients had only vague qualitative idea of the ultimate constituents of matter, since their views were not established on any scientific observations as we know today.

The first of these, was propounded by the French scientist Antoine Lavoisieur (1789). He showed that if a piece of pure tin is heated in a closed vessel so that it combines chemically with the oxygen of the air contained in the vessel, then the total weight of the substances inside the vessel plus the weight of the vessel remained the same before and after the chemical combination. Subsequently, it was established on the basis of many experiments that the total weight of all the components taking part in a chemical reaction is equal to the total weight of all the products of the reaction. This is what is known as the law of conservation of mass.

The law of definite proportions which was propounded by the French chemist J.L. Proust in 1799, states that when two chemical elements A and B combine to produce a chemical compound C (say), then the elements A and B are always present in a definite proportion by weight in the compound C.

For example, the two elements hydrogen and oxygen are always present in the same ratio of 1.008 : 8 by weight in the chemical compound water irrespective of the method of production of water.

The law of multiple proportions was discovered by Dalton himself (1803). When two elements A and B combine chemically to produce two or more chemical compounds, then the ratios of the weights of any one of Atomic and Nuclear Physics 2. them    (say B) which combines with a fixed weight of the other (say A) to form the different compounds C, C are equal to the ratios of integers.

As an example, we know that the elements carbon and oxygen and carbon dioxide (CO,), In one mole of the former (i.e., CO), oxygen; in the latter (i.e., CO,), 12 x 10 kg of carbon is in chemical combine to produce the two chemical compounds carbon monoxide (cn kg of 12x 10 kg of carbon is in chemical combination with 16x 10-3 which are in chemical combination with 12 x 10 kg of carbon above two compounds are in the ratio 16 : 32 or 1: 2.

On the basis of the above laws, Dalton proposed his atomic hypothesis which consisted of the following postulates :

(a) All chemical elements are made up of extremely small particles, known as atoms. The atoms cannot be further subdivided by any chemical process. They retain their identity during chemical reaction.

(b) All atoms of the same element are identical i.e., their weights and other properties are exactly the same. The atoms of different elements differ in their weights and other properties. Each element is characterised by the weight of its atoms.

(c) When different elements combine chemically, it is the atoms of these elements which combine together. This combination of the atoms take place in simple numerical ratios, e.g. 1: 1, 1: 2, 2: 1, 2:3 etc.

With the help of these three postulates, it is possible to explain the laws of chemical combination stated above.

Since the weights and other properties of the atoms of different elements and their numbers taking part in a chemical reaction remain unchanged during the reaction and since the total weight of each element taking part in the reaction is equal to the combined weights of all the atoms present in it, we conclude that the total weight of all the elements taking part in the reaction must be equal to the total weight of alf the substances produced in the reaction.

This is the law of conservation of mass.

Again since chemical compounds are produced by the combination of the atoms of two or more elements in simple numerical proportions and since all the atoms of the same element have equal weights, hence the elements taking part in a chemical reaction always combine together definite proportions by weight.

This is the law of definite proportions.

Now consider the production of two types of chemical compounds C. C, by the combination of the two elements A and B. Suppose that n