ATOM

Long time ago, it was thought that matter is made up of simple, indivisible particles. Greek philosophers thought that, matter could be divided into smaller and smaller particles to reach a basic unit, which could not be further sub-divided.
Democritus (460-370 B.C.) called these particles Atomos, derived from the word “Atomos” means indivisible. However, the ideas of Greek philosophers were not based on experimental evidences.
In the late 17th century, the quantitative study of the composition of pure substances disclosed that a few elements were the components of many different substances. It was also investigated that how, elements combined to form compounds and how compounds could be broken down into their constituent elements.
In 1808, an English school teacher, John Dalton, recognized that the law of conservation of matter and the law of definite proportions could be explained by the existence of atoms. He developed an atomic theory; the main postulate of which is that all matter is composed of atoms of different elements, which differ in their properties.
Atom is the smallest particle of an element, which can take part in a chemical reaction. For example, He and Ne, etc. have atoms, which have independent existence while atoms of hydrogen, nitrogen and oxygen do not exist independently.
The modern researches have clearly shown that an atom is further composed of subatomic particles like electron, proton, neutron, hypron, neutrino, anti-neutrino, etc. More than 100 such particles are thought to exist in an atom. However, electron, proton and neutron are regarded as the fundamental particles of atoms.
A Swedish chemist J. Berzelius- (1779-1848) determined the atomic masses of elements. A number of his values are close to the modern values of atomic masses. Berzelius also developed the system of giving element a symbol.

Evidence of Atoms

It is not possible actually to see the atoms but the nearest possibility to its direct evidence is by using an electron microscope. A clear and accurate image of an object that is smaller than the wavelength of visible light, cannot be obtained. Thus, an ordinary optical microscope can measure the size of an object upto or above 500 nm (l nm = 10-9m). However, objects of the size of an atom can be observed in an electron microscope. It uses beams of electrons instead of visible light, because wavelength of electron is much shorter than that of visible light.
Fig. below shows electron microscopic photograph of a piece of a graphite which has been magnified about 15 million times. The bright band in the figure are layers of carbon atoms.

Electron microscopic photograph of graphite
In the 20th century, X-ray work has shown that the diameter of atoms is of the order 2×10-10 m which is 0.2 nm. Masses of atoms range from 10-27 to 10-25 kg. They are often expressed in atomic mass units (amu) when 1 amu is = 1.661 x 10-27 kg. The students can have an idea about the amazingly small size of an atom from the fact that a full stop may have two million atoms present in it.