The Atomic Theory of Matter

Early Greeks like Democritus first advanced the idea of atomos, the smallest indivisible unit of matter. This concept was later put forth as the atomic theory by the English scientist John Dalton.  

The first postulate of Dalton’s atomic theory suggests that elements are composed of tiny indivisible particles called atoms. For example, hydrogen is made up of two hydrogen atoms bonded together and oxygen is made up of two oxygen atoms bonded together.

The second postulate states that while atoms of the same element are identical, they differ from the atoms of other elements. Thus, all the atoms of hydrogen are identical to each other, but differ from oxygen atoms.  

The third postulate states that atoms of different elements may combine with each other, in simple whole-number ratios, to form compounds. For example, two atoms of hydrogen and one atom of oxygen combine in a 2 to 1 ratio to form H₂O — water. 

The last postulate of Dalton’s theory states that chemical reactions do not change the atoms of one element into atoms of a different element; instead, the existing atoms rearrange themselves to form new substances. During the formation of water, existing hydrogen and oxygen atoms are neither created nor destroyed, they only rearrange. 

Dalton built his atomic theory upon two previous laws of chemical reactions: the law of conservation of mass and the law of definite proportions.

The law of conservation of mass states that the total mass before and after a chemical reaction remains constant. So, 18 g of water breaks down into 2 g of hydrogen and 16 g of oxygen.

The law of definite proportions states that independent of the source, in a given chemical compound, the mass ratio of constituent elements always remains fixed. Thus, a sample of pure water will always have a 1 to 8 mass ratio of hydrogen to oxygen irrespective of the total mass of the water.

Dalton then proposed the law of multiple proportions based on his own theory. According to this law, when two elements combine to form more than one compound, different masses of one element combine with the fixed mass of the other element in a small whole-number ratio. 

Thus, a fixed mass of hydrogen, 2 g, combines with different masses of oxygen, 32 or 16 g, in a small whole-number ratio of 2 to 1, to form two different compounds — hydrogen peroxide, and water.