According to the ancient philosophers, every substance was believed to comprise very small particles, but due to lack of scientific knowledge they could not prove it. John Dalton was the first scientist who propounded the ‘Atomic Theory’ in 1801. According to this 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 ad tom mean cut).
The Structure of an Atom
Atom can neither be created nor be destroyed. Atoms of the same element are similar but atoms of different elements differ. Dalton
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. Protons and neutrons are held together in the nucleus by short range attractive nuclear force.
Electrons revolve around the nucleus in different circular or elliptical orbits. The number of electrons, which ca go in the first orbit is two; in the second, eight; in the third, eighteen; in the fourth, thirty two ad 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 radiations takes place.
Today, it is possible to split the atom. The atom bomb is the result of this new discovery. Splitting of atom is called nuclear fission, and this process is used for generating electric power in atomic power plants.