What Was the Evidence for Dalton’s Atomic Theory?


Martha Robinson

The atomic theory proposed by John Dalton in 1808 revolutionized the way we view the world around us. It provided a scientific explanation for the behavior of matter and helped us understand the properties of different elements. Dalton’s theory was based on several pieces of evidence, which we will explore in this article.

The Law of Definite Proportions

One of the key pieces of evidence for Dalton’s atomic theory was the law of definite proportions. This law states that a given compound always contains the same proportion of elements by mass. For example, water always contains hydrogen and oxygen in a 2:1 ratio by mass.

This observation supported Dalton’s hypothesis that elements combine in fixed ratios to form compounds. He suggested that these ratios were determined by the relative weights of atoms, which he called “ultimate particles”.

Multiple Proportions

Another important piece of evidence for Dalton’s atomic theory was the observation of multiple proportions. This refers to the fact that two elements can combine in different proportions to form different compounds.

For example, carbon and oxygen can combine to form both carbon monoxide (CO) and carbon dioxide (CO2). The ratio of carbon to oxygen is 1:1 in carbon monoxide and 1:2 in carbon dioxide.

This observation supported Dalton’s idea that atoms combine in simple whole-number ratios to form compounds.

Law of Conservation of Mass

The law of conservation of mass was also consistent with Dalton’s atomic theory. This law states that matter cannot be created or destroyed, only rearranged.

Dalton proposed that chemical reactions involve rearrangement of atoms rather than creation or destruction. This idea is now known as the law of conservation of atoms, which is a fundamental principle in modern chemistry.

Brownian Motion

One final piece of evidence for Dalton’s atomic theory came much later, in the form of Brownian motion. In 1827, Robert Brown observed the erratic motion of pollen particles suspended in water.

This observation was later explained by Albert Einstein, who showed that the motion was caused by collisions with water molecules. This provided strong evidence for the existence of atoms and molecules, which were previously thought to be too small to observe.


In conclusion, John Dalton’s atomic theory was supported by several pieces of evidence, including the law of definite proportions, multiple proportions, and the laws of conservation of mass and atoms. These observations provided a scientific basis for understanding the behavior of matter and laid the foundation for modern chemistry.