The Atomic Theory

THE ATOMIC THEORY

The idea that matter is made up tiny indivisible particles called atomos prevailed for more than 2000 years. This was even noted by Robert Boyle in his book The Sceptical Chymist  and by Isaac Newton in his books Principia and Opticks. But it was John Dalton who proposed the Atomic Theory. Though the idea of the atom wasn’t new, Dalton went further by explaining how atoms combine to form compounds. Dalton expressed his ideas in a series of postules:

John Dalton

1.       Matter is made up of tiny indivisible particles called atoms.

2.       Atoms of a given element cannot be converted into atoms of another element.

3.       Atoms of an element are identical in mass and other properties and are different from atoms of any other element.

4.       Compounds result from the chemical combination of a specific ratio of atoms.

The first and second postulates follow the Law of Conservation of Mass, which states that mass is neither created nor destroyed in an ordinary chemical reaction. Since atoms cannot be cut or destroyed or converted into another atom, its mass is conserved. This also rejects the idea of alchemy’s transmutation. The third and fourth postulates support the Law of Definite Proportion and the Law of Multiple Proportion. The Law of Definite Proportion states that different samples of the same compound always contain its constituent elements in the same proportion by mass. It means that water in France is also the same with water in China – both samples contain two atoms of hydrogen and one atom of oxygen. On the other hand, the Law of Multiple Proportion states that if two elements can combine to form more than one compound, the masses that combine with a fixed mass of the other element are in rations of small whole numbers. For example, chlorine can combine with oxygen to form ClO^-, ClO₂^-, ClO₃^-, and ClO₄^-. This shows that the mass ratio of oxygen combined with chlorine in the four compounds is 1:2:3:4. All values are in whole number ratios.

Since John Dalton believed that an atom is indivisible, his picture of the atom was like of a billiard ball – a tiny particle invisible to the naked eye that does not have smaller components.