The most used definition of atom is as follows,
“The smallest particle of an element, which may or may not have an independent existence but can take part in a chemical reaction is called atom.”
Explanation of Define Atom:
For example, the smallest particle of hydrogen, which does not exist independently is called atom of Hydrogen, whereas the smallest part of helium, which exist independently is called atom of helium, which exist independently is called atom of helium. Atoms are represented by the symbols of their elements. For example, atom of Hydrogen is represented by H and that of Helium by He.
An atom is composed of many sub-atomic particles like electrons, proton, neutron, neutrino, anti-neutrino etc. More than 100 fundamental particles are thought to exist in an atom.
The Swedish Chemist J. Berzelius (1779-1848) determined the atomic weights of elements. The value of atomic weights determined by him are close to the modern values. He also developed the system of giving symbols to elements.
Evidence of Atom:
It is difficult to get clear picture of objects, which are smaller than the visible light wavelength, like atoms. Electron microscope is the medium or tool that can be used to visualize an atom to some extent.
Thus an ordinary optical microscope can measure the size of an object up to or above 500nm (1 nm=10-9 m). 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.
In the twentieth century, x-ray work has shown that diameter of atoms are of the order 2X10-10 m which is 0.2mm. Atomic Masses of atoms are normally from 10-27 Kg to 10-25 Kg. They are often expressed in atomic mass unit where 1 amu=1.661X10-27 kg.
The student can have an idea about the amazingly small size of atom from the fact that a full stop may have 2 million atoms present in it.