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Chemical Bonding

Chemical bond is an attractive force which keeps tow atoms or ions together in a molecule.A molecule is formed if it is more stable and has lower energy than the individual atoms. Normally only electrons in the outermost shell of an atom are involved in bond formation and in this process each atom attains a stable electronic configuration of inert gas. Atoms may attain stable electronic configuration in three different ways by losing or gaining electrons by sharing electrons. The attractive forces which hold various constituents (atoms, ions etc) together in different chemical species are called chemical bonds. Elements may be divided into three classes.

  • Electropositive elements, whose atoms give up one or more electrons easily, they have low ionization potentials.

  • Electronegative elements, which can gain electrons. They have higher value of electronegativity.

  • Elements which have little tendency to loose or gain electrons.

Three different types of bond may be formed depending on electropositive or electronegative character of atoms involved.
Electropositive element + Electronegative element = Ionic bond (electrovalent bond)
Electronegative element + Electronegative element = Covalent bond
or less electro positive + Electronegative element = Covalent bond

Electropositive + Electropositive element = Metallic bond.

  • The Lewis Theory

The octet rule:- The Lewis theory gave the first explanation of a covalent bond in terms of electrons that was generally accepted. If two electrons are shared between two atoms, this constitutes a bond and binds the atoms together. For many light atoms, a stable arrangement is attained when the atom is surrounded by eight electrons.

This octet can be made up from some electrons which are totally owned and some electrons which are ‘shared’. Thus atoms continue to form bonds until they have made up an octet of electrons. This is called the ‘octet rule’. The octet rule explains the observed valencies in a large number of cases. There are exceptions to the octet rule; for example, hydrogen is stable with only two electrons. Other exceptions are discussed later.Today, the conventional Lewis structure representation of a pair of bonded electrons is by means of a ‘dash’ (-) usually called a ‘bond’. Lone pairs or ‘non-bonded’ electrons are represented by ‘dots’. Some structures are represented below:

It is therefore useful to remember some important categories of exceptions:

  1. Odd-electron species like NO, NO2, O2 etc. Since it is impossible to distribute an odd number of electrons into pairs, these species necessarily violate the octet rule.

  2. The Valence Bond theory handles such species rather clumsily (odd-electron bonds etc.) the molecular orbital theory copes much better.

  3. Species in which the central atom `expands' its octet (so to speak) PCl5, SF6, many transition metal compounds etc.

  4. Electron -deficient species like BCl3, BeCl2, AlCl3, B2H6 etc. in which the central atom has fewer than eight electrons.

We will discuss the concepts of chemical bonding for IIT JEE under following subtopics in more detail

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