11 Class Chapter 4- Chemical Bonding and Molecular Structure

Chemical Bonding and Molecular Structure

Attractive force which holds various constituents (like atoms, ions etc.) together in difference chemical species is known as a chemical bond.

Kossel Lewis Approach to Chemical Bonding: -

                                                                        Lewis discussed that atoms achieve stable octet when they linked by chemical bonds.

·        In NaCl, an electron transfer from Na to Cl and give Na⁺ and Cl^- ions and,

·        In case of Cl₂, H₂, F₂, sharing of electron takes place to complete stable outer octet.

·        Valence electrons take part in chemical reaction or combination.

·        Symbol to represent valence electron in atom is known as Lewis's symbol.

Example: -

                   

·        Significance of Lewis symbol: - 

To calculate valence = (no. Of dots and or / eight minus no. Of dots)

Octet rules: - 

                    According to this rule, atoms can combine either by sharing electron or by transfer (gaining or losing) of electron to complete octet (i.e., 8 electrons in outer most shell).

Covalence bond: -   

                          Bond forms by sharing of electrons.

·        When sharing of one pair of electron takes place is known as single covalent bond.

·        When sharing of two pair of electrons takes place is known as double bond.

·        When sharing of three pair of electron takes place is known as triple bond.

Formal charge (F.C.):  - 

For polyatomic molecule or ion, formal charge of an atom in a Lewis structure = [Total no. of                 Valence electron in free Atom] - [total no. of nonbonding (lone- pair) elements]- 1/2[total no of bonding (shared) electrons]

Ex - Lewis's structure of O₃ (ozone molecule)

                                

Atoms in O₃ is marked as 1,2,3. 

·        Central O atom marked as 1.

·        Central O atom marked as 2.

·        End O atom marked as 3.

You are learning chemical bonding and molecular structure at our chemistry notes website www.ChemistryNotesInfo.com

Chemical Bond: 

Chemical bond is defined as a force that acts between two or more atoms to hold them together as a stable molecule.

• The phenomenon of union of two or more than two atoms involve redistribution of electrons so that each atom involved in bonding acquire stable configuration in order to gain stability is known as ‘chemical bonding’.
• The combination of atoms in bonding involves a decrease in potential energy. As a result of this, net attraction is developed between bonded atoms.

Valency: 

(derived from ‘Latin’ word ‘Valentia’): The combining capacity of atom is known as valency. 

Old concept of valency: 

The total number of Hydrogen (H) or Chlorine (Cl) atoms with which atoms can combine is known as its valency or combining capacity.

Molecule	Valency
H2O	O: 2
NH3	N: 3
MgCl2	Mg: 2
AlCl3	Al: 3

Special point: According to this old concept, an element may have multiple valences and fractional valences in some compounds.

Molecule	Valency of N
NH3	3
N2H4	2 = 4/2
N3H	1/3

Valency of an element is also described in terms of number of oxygen atoms with which it combines.

According to this old concept, moving from left to right in a period, valency w.r.t. Hydrogen increases from 1 to 4 and then decreases from 4 to 1 with-respect-to oxygen, it increases from 1 to 7.

Group	I	II	III	IV	V	VI	VII
Valency w.r.t. hydrogen	1	2	3	4	3	2	1
Hydrides	RH	RH2	RH3	RH4	RH3	RH2	RH
Example	NaH	MgH2	AlH3	SiH4	PH3	H2S	HCl

Group	I	II	III	IV	V	VI	VII
Valency w.r.t. oxygen	1	2	3	4	5	6	7
oxides	R2O	RO	R2O3	RO2	R2O5	RO3	R2O7
Example	Na2O	MgO	Al2O3	SiO2	P2O5	SO3	Cl2O7

Valence electron: Outermost shell electrons are known as valence electrons. They take part in chemical bonding.

Modern concept of valency: 

The number of chemical bonds formed by an atom in a molecule is known as its valency.

Electronic theory of valency: 

Valency of an element depends upon number of valence electrons present in an atom.

• If eight electrons are present in outermost shell, then it is highly stable configuration (inert gas except ‘He’)
• Each atom having less than eight electrons in valence shell has a tendency to achieve its stable configuration (octet). In order to complete its octet, it may lose or gain or donate or accept or share required number of electrons.
• Valency of an element = number of electrons gain or lose or shared by an atom.