Recapitulation :

• In the 6th century B.C., an Indian philosopher, Kanad, propounded the theory that everything is made of ’paramanu’.
• Atom is the smallest particle of an element. Atom takes part in physical and chemical processes.
• Dalton proposed his atomic theory suggesting that atoms of a given ele- ment are all alike while atoms of different elements are dissimilar.
• Rutherford carried out α –particle scattering experiment and suggested the presence of the positively charged centre called ’nucleus’ in the atom.
• Bohr systematized the concepts known at that time and put forth his own ideas to give a revolutionary model of atom which is used even today.
• Chadwick discovered electron in 1932.
• Atom has three elementary or fundamental particles in it – electrons, pro- tons and neutrons. Protons are positively charged, electrons are negatively charged while neutrons are neutral. The mass of the electrons is negligible. The mass of neutron is almost equal to that of a proton.
• Atom consists of a central part called nucleus and a number of electrons revolving around it in its orbits.
• Electrons revolve around the nucleus in certain selected orbits called stationary orbits. They are given numbers 1,2, 3 4 ...... and denoted by K,L, M, N ... ... and so on.
• Electrons occupy shells in the increasing order of energy.
• Electrons are distributed in shells or orbits following certain rules. The maximum number of electrons in any shell is given as 2n2 where n is the number of the shell. The outermost shell of any atom can have maximum 8 electrons i.e. octet.
• The outermost shell of an atom is called valence shell and the electrons in the valence shell are called valence electrons.
• The number of electrons lost, gained or shared by an atom to complete its valence shell is called the valency of the element.
• There are two types of valency – electrovalency and covalency