Atoms of different elements combine in fixed ratio to form compound.....

The Postulate of Dalton's Atomic Theory Explaining the Law of Definite Proportions

The law of definite proportions states that a given chemical compound always contains the same elements in the same proportion by mass. This law can be explained by one of the postulates of Dalton's atomic theory, which states that atoms combine in simple, whole-number ratios to form compounds.

The Law of Definite Proportions
The law of definite proportions, also known as the law of constant composition, was first formulated by French chemist Joseph Louis Proust in the late 18th century. It states that in a chemical compound, the elements are always present in a fixed proportion by mass. For example, water (H2O) always consists of two hydrogen atoms and one oxygen atom, and the mass ratio of hydrogen to oxygen is always 2:16, or 1:8.

Dalton's Atomic Theory
John Dalton, an English chemist, further developed the concept of the law of definite proportions with his atomic theory in the early 19th century. Dalton proposed several postulates to explain the behavior of atoms and their interactions:

1. Atoms are indivisible and indestructible: Dalton postulated that atoms are the fundamental building blocks of matter and cannot be divided or destroyed during chemical reactions.

2. Atoms of the same element are identical: He stated that all atoms of a particular element are identical in mass and chemical properties.

3. Atoms combine in simple, whole-number ratios: This postulate directly relates to the law of definite proportions. Dalton proposed that atoms combine with each other in fixed ratios to form compounds. For example, if element A combines with element B to form a compound, he suggested that the ratio of A to B would always be a simple whole number, such as 1:1, 1:2, or 2:3.

Explanation of the Law of Definite Proportions
Based on Dalton's postulate of atoms combining in simple, whole-number ratios, the law of definite proportions can be explained as follows:

When elements combine to form a compound, their atoms join together in specific ratios determined by their atomic masses. The ratio of the masses of the elements in a compound is fixed and constant, resulting in a consistent proportion of elements by mass. This means that regardless of the amount of compound being analyzed, the ratio of the elements remains the same.

For example, consider the compound carbon dioxide (CO2). According to Dalton's atomic theory, one atom of carbon combines with two atoms of oxygen. The atomic masses of carbon and oxygen are 12 and 16, respectively. Therefore, the mass ratio of carbon to oxygen in carbon dioxide is 12:32, or 3:8.

This consistent ratio is observed in any sample of carbon dioxide, whether it is a small or large quantity. It demonstrates the law of definite proportions, as the proportion of carbon to oxygen by mass remains constant.

In conclusion, the postulate of Dalton's atomic theory that states atoms combine in simple, whole-number ratios provides an explanation for the law of definite proportions. It states that the mass ratios of elements in a compound are fixed, resulting in a consistent proportion by mass. This fundamental concept helps us understand the behavior of chemical compounds and their composition.