In the fifth century B.C, the Greek philosopher Democritus expressed the belief that all matter consists of very small, indivisible particles, which he named atomos (meaning uncuttable or indivisible). Although Democritus’ idea was not accepted by many of his contemporaries (notably Plato and Aristotle), somehow it endured. It was in 1808 that an English scientist and schoolteacher, John Dalton, formulated a precise definition of the indivisible building blocks of matter that we call atoms.

Dalton’s atomic theory

Dalton’s work marked the beginning of the modern era of chemistry. The hypotheses about the nature of matter on which Dalton’s atomic theory is based can be summarized as:

1. Matter consists of very small particles called atoms.
2. Atoms are indivisible.
3. All atoms of a given element are identical, they have the same mass and properties.
4. Atoms of one element are different from atoms of other elements in mass and other properties.

Shortcomings of Dalton’s atomic theory

The following two statements in Dalton’s atomic theory were wrong:

1. Atoms are indivisible: This statement is proved to be wrong because atoms can be divided into subatomic particles.
2. Atoms of the same element have the same mass: Atoms of the same element may not have the same mass as explained in the modern atomic theory. These facts will be explained in the modern atomic theory.

The modern atomic theory

The following statements describe the main points of the modern atomic theory.

1. An element is made of atoms. An element is composed of small particles called atoms.
2. Atoms are built up from small particles called protons, electrons and neutrons. Thus atoms are divisible and are not the smallest particle of a substance.
3. All atoms of the same element have the same number of protons (and electrons) but may have different number of neutrons. This is due to the existence of isotopes of elements. However atoms of the same element have identical chemical properties. We will discuss about isotopes in the next section.
4. Atoms of different elements are different because of the different number of protons. Hence they have different chemical properties.