1.   It is well known fact that except noble gases, no element exists as independent atoms under ordinary conditions.
2.   Most of these elements exist as molecules which are cluster or aggregates. How do atoms combine to form molecules and why do atoms form bonds? Such doubts will be discussed in this chapter.
3.   A molecule will only be formed if it is more stable and has a lower energy, than the individual atoms.

Chemical bond

Chemical bond is an attractive force which keeps two atoms or ions together in a molecule. A molecule is formed if it is more stable and has lower energy than the individual atoms. We all know that except for noble gases, the atoms of no other element exist independently. A molecule may have the same or different atoms. Atoms combine with other atoms to gain stable electronic configuration.
 Atoms attain stable electronic configuration in three different ways
1. By losing electrons.
2. Gaining electrons
3. By sharing electrons.
The attractive forces which hold various constituents (atoms, ions etc) together in different chemical species are called chemical bonds.

Types of chemical bonds

Electrovalent bond: it is formed due to transfer electrons from one atom to another atom.
Covalent bond: It is formed by the sharing of electrons between the two atoms.

Tendency to acquire noble gas configuration

(a) The first explanation about the cause of combination between atoms is based upon the understanding of electronic configuration of noble gases.
(b) It has been observed that atoms of noble gases have little or no tendency to combine with each other or with atoms of other elements.
(c) This means that these atoms must be having stable electronic configurations.

Lewis octet rule

(a) Except helium, whose electronic configuration is 1s2, the s and p subshells of the highest energy level contain a total of eight electrons.
(b) It is therefore concluded that ns2np6 configuration in the outer energy level constitutes a structure of maximum stability and therefore, of minimum energy.
(c) The tendency of atoms to achieve eight electrons in their outermost shell is known as Lewis octet rule.


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