According to the ancient philosophers, every substance was thought to be made up of very small particles. But because of the lack of scientific knowledge they could not prove it through experiments. John Dalton was the first scientist who propounded the atomic theory in 1803. According to his theory, every element is made up of very small particles called atoms. Atom is a Greek word which means “that cannot be cut” (‘A’ means ‘not’ and ‘tom’ means ‘cut’). Atom can neither be created nor destroyed. Atoms of the same element are similar but atoms of different elements are dissimilar. All these arguments of Dalton have been proved wrong by the modern researchers. It is now an established fact that atom is not the smallest particle of matter. In fact, it is made up of still smaller particles.
The structure of the atom can be compared to our solar system. Like the planets revolving around the sun, the negatively charged electrons revolve around the positively charged nucleus in different orbits. Almost the entire mass of the atom is concentrated in the nucleus. The nucleus is made up of two types of particles called ‘protons’ and ‘neutrons’.
Protons are positively-charged particles, while neutrons are neutral particles. The mass of the proton is almost equal to the mass of the neutron. Protons and neutrons are held together in the nucleus by short range attractive nuclear forces.
Electrons revolve around the nucleus in different circular or elliptical orbits. The number of electrons which can go in the first orbit is two, in the second eight, in the third eighteen, in the fourth thirty-two and so on. The electrons in the outermost orbit are called valence electrons. The properties of any element depend upon the number of valence electrons. When energy is given to an atom, the electrons in the outermost orbit absorb it and are excited to the higher orbits. When they fall back to their original orbit, emission or radiation takes place.
Today it has become possible to split the atom. Atom bomb is the result of this new discovery. Splitting of atom is called ‘nuclear fission’ which is also used for generating electric power in atomic power plants.
