Core Tip

• In the 6th century B.C., an Indian Philosopher, Kanad, propounded a theory stating that everything is made of ’paramanu’ (atom.)
• The smallest particle of an element is atom which takes part in physical and chemical processes.
• In 1808, Dalton put forth his atomic theory stating that atoms of a given element are all identical and atoms of different elements are different.
• In 1897, Thomson discovered electron and put forth his model of atom similar to Christmas pudding.
• In 1911, Rutherford carried out scattering experiment using α –particles. He put forth his model of atom suggesting that at the center of the atom there is a positively charged center called nucleus.
• In 1913, Bohr proposed his model of atom retaining most of the assumptions of Rutherford but corrected the drawbacks in his model.
• In 1932, Chadwick discovered neutron.
• The protons, electrons and neutrons are called elementary or sub–atomic particles. They have their characteristics mass and charge.
• Atom consists of a central part called nucleus and a number of electrons revolving around it in stationary orbits or shells.
• Electrons occupy shells in increasing order of energy.
• Electrons are distributed in shells following certain rules.
• The outermost orbit of an atom is called ’valence shell’ and the electrons in it are called ’valence electrons’
• The combining capacity of one atom with other atoms is called valency. There are two types of valencies –electrovalent and covalent.

It would be clear to the students that the concept of Atom is not new. However, identification of exact structure within an atom has not been easy to arrive at. Grouping of the elements has been done based on a set pattern that some elements show in terms of number of electrons in the outer shell and often dictate the reactions of that element with some other elements. This is an important point to note.