Electronic Theory of Valency

Valence shell

The outermost shell of an atom of an element is called the valence shell and the electrons present in this shell are called valence electrons. Except for valence electrons, the rest electrons of an atom are called core electrons.

Lewis electron dot symbol

Valence electrons are shown in the symbol by dot (.).

Postulates of Electronic theory of valency

1. The valence electrons take part in the formation of a covalent bond.
2. Atoms with 8 electrons (octet rule) in their valence shell are most stable. However, hydrogen and helium satisfy the duplet rule.
3. Atom combines with each other by loss or gain of electrons or by mutual sharing of electrons.
4. Each combining atom attains the electronic configuration to the nearest inert gas by loss or gain or mutual sharing of electrons.
5. The number of electrons lost or gained by an atom to get the nearest inert gas configuration is called the valency of the corresponding element.
6. The chemical bond is formed if there is an overall decrease in energy. Lower the energy, stable the molecule.

Types of bond

A. Electrovalent or Ionic bond

This bond is formed by the complete transfer of one or more electrons from one atom to another. It takes place between the metal and non-metal. The metal loses an electron and forms a cation whereas the non-metal gains an electron and form anion. Cations and anions come close by the electrostatic force of attraction. The compounds formed by the electrovalent or ionic bond are called ionic or electrovalent compounds. The number of electrons lost or gained by an atom of an element is called the electrovalency of the element.
Examples of ionic bond:

1. Formation of NaCl

2. Formation of MgCl₂

3. Formation of Al₂O₃

4. Formation of CaO

Characteristics of ionic compounds

1. They exist as solid in ordinary condition.
2. They are generally soluble in water.
3. They are usually hard and brittle.
4. They usually have high melting and boiling points.
5. They are non-directional.
6. Ionic solid can’t conduct electricity. They conduct electricity in the molten or aqueous state as they contain mobile ions.

B. Covalent bond

The bond formed by the mutual sharing of electrons between combining atoms of the same or different elements is called a covalent bond. It takes place between non-metals. It is represented by a dash (-). The number of electrons that an atom contributes for sharing during covalent bond formation is called covalency. The compounds formed by the covalent bond are called covalent compounds.
Examples of covalent bonds:

1. Formation of hydrogen molecule

2. Formation of chlorine molecule

3. Formation of oxygen molecule

4. Formation of nitrogen molecule

5. Formation of hydrogen chloride molecule

Lewis structure of some covalent compounds

Characteristics of covalent compounds

1. They may exist in solid, liquid or gas state.
2. Generally, they don’t conduct electricity.
3. They are generally insoluble in water.
4. They have low melting and boiling points than ionic compounds.
5. They are directional in nature.

C. Coordinate covalent bond

The bond formed by the sharing of electrons in which the shared pair of electrons is contributed by one of the bonded atoms is called a coordinate covalent bond. The atom that donates the shared pair of electrons is called the donor atom and which accepts the shared pair of electrons are called the acceptor atom. This bond is also called a semi-polar or dative bond. It is represented by an arrow ( → ) pointing from donor atom to acceptor atom.
Examples of co-ordinate covalent bond:

1. Formation of ammonium ion ( NH₄⁺ )

2. Formation of hydronium ion ( H₃O⁺ )

Lewis structure of some coordinate covalent compounds

Exception of octet rule

1. Molecules having incomplete octet:
The compounds of Be, B and Al after sharing of electrons possess six (in B) or four (in Be) electrons in the valence shell of the central atom. eg. BeCl₂, BCl₃, AlCl₃ etc.

2. Molecules having super octet:
Many compounds of P, S, Cl, Br, I, etc. contain more than eight electrons after bond formation.

Polarity of covalent bond: Ionic character of covalent bond

When a covalent bond is formed between the atoms of different elements, the element having higher electronegativity attracts the shared electrons more towards itself. The more electronegative atom acquires a partial negative charge and the less electronegative atom acquires a partial positive charge. Such bonds are called polar covalent bonds. eg. In HCl, chlorine is more electronegative than hydrogen, so shared electron shift towards chlorine, and hence chlorine bears partial negative charge whereas hydrogen bears a partial positive charge.

H^{\delta +} - Cl^{\delta -}

Some examples of polar bonds are H-Cl, H-Br, O-H, C-Cl, C-Br, C-O, C-N, N-H, etc.

Some Important Questions

1. Define ionic, covalent, and coordinate bonds with two examples of each.
2. Write the Lewis structure of NH₄NO₃, CaCO₃, H₂SO₄, P₂O₅, C₂H₅OH.
3. What do you mean by polar covalent bond?
4. What is a lone pair of electrons? How many lone pairs of electrons are present in each chlorine atom of chlorine molecules?
5. What are valence electrons? Define the valency of an element in terms of valence electrons.