Dalton's atomic theory

From time immemorial, philosophers were puzzled about the nature of matter. They were suggesting that all matter is composed of tiny, discrete , indivisible particles called atom. In 1808, John Dalton put forward the atomic theory and successfully explained the laws of chemical combination known at that time. Dalton's atomic theory may be summarized in the following postulates:

1. All matter consists of extremely small indivisible particles called atoms.

2. Atoms of same element are all alike.

3. Atoms of different elements are entirely different and have different properties.

4. Atoms cannot be destroyed, created or transformed into atoms of other elements.

5. Atoms combine together in simple whole number ratio to give compounds.

6. The relative number and kinds of atoms are constant in a given compound.

Dalton's atomic theory is the milestone in the development of modern chemistry. Dalton's original statement has undergone expansion, modification and clarification to a great extent. According to dalton atoms unite together to form chemical compounds. The quantitative weight relationship in a chemical reaction is very important. Stoichiometry is the branch of chemistry which deals with the weight relationship in chemical reaction and weight relationship that prevails in a chemical compound. This type of study was initiated by Antoine Lavoisier. Lavoisier was the man who introduced chemical balance in chemistry laboratory. The quantitative relationship in chemical combination is governed by the following laws of chemical combination or stoichiometry.

1. The law of conservation of mass

2. The law of definite proportions

3. The law of multiple proportions

4. The law of reciprocal proportions

5. The law of gaseous volumes

In the above five laws , the fifth law is not concerned with the weight relationship, rather it deals with volume relationship in a chemical reaction.

The law of conservation of mass:

The law of conservation of mass was probably first stated by russian chemist M.V. Lomonosov in 1756. This law was again independently restated by the French chemist Antoine Lavoister in 1774. The law of conservation of mass may be stated as

Mass is neither gained nor lost during a chemical reaction or in a chemical reaction the total mass of the reactants consumed is equal to the total mass of products formed.

This law is also known as the law of indestructabilityof matter . The truth of this law had been firmly established by German scientist H.Landolt by performing a series of experiments during the period of 1900-1908. Landolt found no deviation from the law within a possible error of 1 part in 10,000,000.

The law of definite proportions:

The law of definite proportions is also known as law of constant proportion. This law first enunciated by French chemist Joseph Louis Proust in 1799 which states that:

The same chemical compound always contains same elements combined together in definite proportion by weight regardless of the origin or mode of formation of the compound.

This law may be illustrated by taking the following examples.In certain compounds , the law of definite proportion is violated. For example, the metal zinc forms only one normal oxide, namely zinc oxide, but under certain conditions this compound is found to have less than stoichiometry proportions of oxygen, so that formula for zinc oxide  becomes different.

 The law of multiple proportions:

When one element combines with another element to give only one compound., then these elements combine in definite proportions. Sometimes, two elements combine to give two or more compounds. In such cases the proportions of the two elements will be naturally different in two different compounds. For example, carbon combines with oxygen to give two compounds, namely carbon carbon dioxide and carbon monoxide. The ratio by weights of carbon and oxygen in these two compounds are different, but there is a definite relationship in the weights of oxygen which combine with the constant weight of carbon. After studying composition of innumerable compounds formed by the same two elements, John Dalton in 1803 put forth the following law known as the  law of multiple proportions which may be stated as:

When one element combine with another element to formtwo or more dufferent compounds , then the weights of one of the elements which combine with theconstant weught of the other bear  a simple whole number ratio to one another.

The law of reciprocal proportions:

The law of reciprocal proportions tells us that there is a definite relationship between the weights of elements in any given compound. Now the question is," Is there any simple relationship between the weights of two elements each of which combines with the fixed weight of third element?" To answer this question, let us study the combine proportions of the elements in two compounds water and hydrogen chloride.

Such relation was first noticed by Richter . After studying such reciprocal relationship that existed in a large number of compounds . Richter in 1792 stated the following law known as The law of reciprocal proportions. This law states that:

When two different elements combine separately with the same weight of third element , the ratio in which they do so will be the same or some simple multiple of the ratio in which they unite with each other.

The law of gaseous volumes:

This law deals with the relationship between the volumes of gaseous reactants and products. Gay Lussac had stated this law in 1805. It states that:

Whenever gases react, they do so in volumes which bear a simple ratio to one another, and to the volumes of the products if these are also gases provided all measurements are made under similar conditions of temperature and pressure.

It should be borne in mind that all the laws of chemical combination given above were established in the second half of the eighteenth century and the early years of the nineteenth century before John Dalton gave his atomic theory. These law were formulated purely on the basis of experimentations on the chemical analysis and synthesis of compounds.