Chapter 1 - BASIC CONCEPTS

what is an atom? brief history of atom.

Definition: Atom is defined as smallest particle of an element which may or may not exist independently. e.g He, Ne, Ar etc. but Hydrogen, Oxygen atoms cannot exist independently.

The modern researches have clearly shown that atom is further composed of subatomic particles like electrons, protons, neutrons, hyperons, neutrino, anti-neutrons etc. more than 100 particles are present in atom.

history of atom

Greek philosopher thought that when matter is broken into smaller and smaller particles, finally a smallest particle is obtained which cannot be further subdivided. Democritus called these particles exiled atoms. This term has been derived from word "atomis" which means indivisible.

In late 17^th century it was found that atoms of same element are present in different substances. It was also discovered how atoms of different elements combine to form compounds and how to break a given compound into its elements.

atomic theory of matter

Dalton showed that law of conservation of matter and law of constant proportions could be explained on the basis of atoms. He developed an atomic theory according to which all matter is composed of atoms of different elements differ in their properties.

J. Berzelius determined atomic masses of elements. Berzelius also developed a system of giving symbols to each element.

EVIDENCE OF ATOMS

It is not actually possible to see atoms. An optical microscope can measure size of an object of size 500nm or above. The size of an atom is almost 0.2nm. The objects of size of an atom could be observed in an electron microscope. It uses a beam of electrons instead of visible light. The wavelength of electrons is much shorter than that of visible light. An electron microscope photograph of a piece of graphite is shown over here. The bright bands in fig are layers of c-atoms.

SIZE AND MASS OF ATOM:

In twentieth century x-ray work has shown that the diameter of atoms are of the order of 2x10 ^-10 meter or 0.2nm.

Masses of atoms range from 10^-25 kg to 10^-27 kg. The masses of atom are generally expressed in terms of amu. 1 amu = 1.661x10^-27 kg. A full stop "." can bear two million atoms present in it. It gives an idea about very small size of an atom.