Chemistry Revision of Chemical Bonding For NDA Part-2
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Types of Chemical Bond

There are three types of chemical bond

1. Electrovalent or ionic bond

2. Covalent bond

3. Coordinate bond

Electrovalent or Ionic Bond

`=>` The chemical bond formed by transfer of electron from a metal to a non- metal is known as ionic or electrovalent bond.

`=>` It is formed between most electropositive elements i.e. metals (like, Na, K, Ca, Ba, Mg etc.) and most electronegative elements i.e. non-metals (like `O`, `F`, `N`, `S`, `Cl`, `Br` etc.).

`=>` The number of electrons lost or gained by the element is called electrovalency of the element which is equal to charge of cation or anion respectively and these types of compounds are called electrovalent compounds.

`=>` Cations are formed by losing electrons by electropositive elements and anions are formed by taking up electrons by electronegative elements.

`=>` Bond is generally ionic, if the difference in electronegativity of the atoms is around `1.9` and more.

Characteristics of Ionic Compounds

The main characteristics are as following

(i) Crystalline : Their constituent units are ions. These ions are arranged together in a regular way in an ionic lattice. e.g. in `NaCl` lattice each sodium ion is bonded with six `Cl^-` ions and each `Cl^-` ion is surrounded by six `Na^+` ions.

(ii) Hard and Brittle : These are hard and brittle in nature due to the strong force of attraction between oppositely charged ions. These solids have low volatility, high stability and high density.

(iii) High Melting and Boiling Points : Because ionic bonds are very strong due to presence of strong electrostatic force between ions.

(iv) Conductive when liquid : Ionic compounds can only conduct electricity if their ions are free to move. Ionic compounds do not conduct electricity when they are solid. But these compounds conduct electricity when dissolved in water or melted.

(v) Solubility : These are soluble in polar solvents like water but insoluble in non-polar solvents like `CCl_4`, `C_6H_6` etc.

(vi) Ionic compounds form ions in solution, hence exhibit ionic reactions which are quite fast and instantaneous.

(vii) Ionic bond is non-rigid and non-directional, so these do not exhibit isomerism.

Variable Electrovalency

The elements which change their valency and have more than one electrovalency are said to possess variable electrovalency. This is due to following two reasons

(i) Inert Pair Effect : It is observed for heavy elements like Pb, Bi, etc. `ns^2` electrons have extra stability and hence do not take part in bond formation. This effect is called inert pair effect. This effect increases with increasing the atomic number in a group.

e.g. Ge and Pb belongs to the 14th group of Periodic Table, but Ge stable in `Ge^(4+)` state and Pb stable in `Pb^(2+)` state. Thus, `Pb^(2+)` salts are stable than `Pb^(4+)` salts.

(ii) Unstable configuration of the core : In d-block elements, core possess unstable arrangement of electrons. Hence, they show variable valency. e.g. `Fe^(2+) (3s^2 , 3p^6, 3d^6)` lose an electron to change into `Fe,^(3+)` which cannot lose further electrons.

 
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