Chemistry (NTSE/Olympiad)  

2. Elementary Idea of Bonding


Electrovalency :
When an element forms electrovalent bond, its valency is known as electrovalency.
The number of electrovalent or ionic bonds an atom can form is called its electrovalency. The electrovalency of an element is, therefore, equal to the number of electrons lost or gained by the atom to form an ion.
Elements which lose electrons show positive electrovalency and those which gain electrons show negative electrovalency. For example, in the formation of sodium chloride (Na+Cl) , the electrovalency of sodium (Na) is +1, while that of chlorine (Cl) is – l.
Elements which lose or gain one, two, three, ... , etc., electrons are said to be monovalent (or univalent), divalent (or bivalent), trivalent, ... , etc., respectively.
Monovalent elements : Na, CI, F
Divalent elements : Mg, Ca, Ba, O
Trivalent elements : Al, B
Characteristics of electrovalent or ionic compounds :
1. Electrovalent compounds are made up of positively and negatively charged ions. For example, sodium chloride (NaCl) is made up of Na+ and Cl ions arranged in a definite order in three dimensions to form crystals.
2. Electrovalent compounds have high melting and boiling points. This is due to the presence of strong electrostatic forces of attraction between the positive and negative ions. A large amount of heat energy is required to break this force of attraction. Hence, the melting and boiling points of electrovalent compounds are high.
3. Electrovalent compounds are usually soluble in water but insoluble in organic solvents such as benzene, acetone, carbon disulphide and carbon tetrachloride.
4. Electrovalent compounds conduct electricity in molten state and in their aqueous solutions.
In solid electrovalent compounds the ions are held together in fixed positions and cannot move. Hence, such compounds in the solid state do not conduct electricity.
When an electrovalent compound is dissolved in water or is melted, the crystal structure breaks down. The ions now become free to move and can, therefore, conduct electricity.
That the ionic compounds in molten state or in solution become conductors of electricity.

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