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class 11 ncert chemistry chapter 4
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UNIT 4 CHEMICAL BONDING AND MOLECULAR STRUCTURE The attractive force which holds various constituents together in different chemical
species is called a chemical bond.
Kossel-Lewis Approach to Chemical Bonding
Kossel and Lewis explained the formation of chemical bonds in terms of electrons.
Atom is a positively charged ‘Kernel’ (Nucleus + Inner electrons).
The outer shell could accommodate a maximum of eight electrons.
These eight electrons occupy the corners of a cube which surround the kernel.
Eg: In Sodium, the single outer electron would occupy one corner of the cube.
In noble gases all the eight corners would be occupied.
LEWIS SYMBOLS
G.N Lewis introduced simple notations to represent valence electrons in an atom.
These notations are called Lewis symbols or Lewis notations.
According to Lewis symbols, the symbol of the element represents the nucleus.
The valence electrons are represented as dots () or crosses (x) around the symbol.
The valency of Li, Be, B and C are 1, 2, 3, and 4 respectively.
The valencies of N, O, and F are 3, 2, and 1 respectively.
SIGNIFICANCE OF LEWIS SYMBOLS
The number of dots around the symbol represents the number of valence electrons.
This number of valence electrons helps to calculate the common or group valency of the
element.
The group valency of the elements is either equal to the number of dots in Lewis
symbols or 8 minus the number of dots or valence electrons.
OCTET RULE
Atoms can combine either by transfer of valence electrons from one atom to another or
by sharing of valence electrons in order to have an octet in their valence shells.
This is known as Octet Rule.
IONIC BOND OR ELECTROVALENT BOND
When an electronegative element combines with an electropositive element, one
or more electrons are transferred from the valence shell of the electropositive atom to
the valence shell of the electronegative atom.
As a result, the electropositive atom becomes a +ve ion and the electronegative atom
becomes a ―ve ion.
The electrostatic force of attraction between positive and negative ions is called
electrovalent bond or ionic bond.
PREPARED BY HAIZEL G. ROY, HSST (HG) CHEMISTRY, GOVT. H.S.S. KALAMASSERY, ERNAKULAM Page | 1
, Ionic bond may also be defined as the electrostatic force of attraction holding the
oppositely charged ions.
Eg: Formation of NaCl
Na Cl Na Cl―
Na Cl―
NaCl
Formation of CaF2 may be shown as
Ca Ca2 2 e―
F e―
F―
Ca2 2 F―
CaF2
ELECTROVALENCY
The number of electrons gained or lost by an atom during the formation of an Ionic
bond is called electrovalency of the atom.
Electrovalency is equal to the charge on the atom.
ENERGY CHANGES DURING THE FORMATION OF AN IONIC BOND
Formation of gaseous cation: A(g) IE A
e―
The energy required for this step is called Ionisation energy.
Formation of gaseous anion: B(g) e―
B― EA
The energy released during this step is called Electron affinity.
The packing of positive and negative ions to form the solids
The energy released in this step is called Lattice Energy.
FACTORS FAVOURING IONIC BOND FORMATION
Low Ionisation Energy of the electropositive atom.
High Electron Affinity of the electronegative atom.
High Lattice Energy of the compound formed.
LATTICE ENERGY
The amount of energy released when one-gram mole of an ionic solid is formed from
gaseous ions.
It is represented as A (g) B―(g)
A B―(S) Lattice Energy
LATTICE ENTHALPY
It is defined as the energy required to completely separate one mole of a solid ionic
compound into gaseous constituent ions.
The lattice enthalpy is equal to lattice energy with reverse sign.
CONSEQUENCE OF LATTICE ENERGY
The greater the lattice energy, the more stable is the ionic compound.
Ions having higher charge and smaller radii will have greater lattice energy.
PREPARED BY HAIZEL G. ROY, HSST (HG) CHEMISTRY, GOVT. H.S.S. KALAMASSERY, ERNAKULAM Page | 2
, The value of lattice energy affects the solubilities of ionic compounds.
COVALENT BOND
A covalent bond is formed by the mutual sharing of electrons between atoms.
The atoms involved in the covalent bond formation contribute equal number of electrons
for sharing.
The shared electrons become common to both the atoms.
This shared pair of electrons is called bond pair.
If the atoms shared one pair of electrons, the bond formed is called a single covalent
bond.
If the atoms shared two pairs of electrons, the covalent bond is called a double bond.
If the atoms shared three pairs of electrons, the bond is called a triple bond.
Cl Cl ― Cl
Cl or Cl ― Cl
N N
N N or N N
O O═O
O or O═O
COVALENCY
The number of electron pairs shared by an atom or the number of electrons contributed
by an atom for the formation of covalent bonds is called covalency.
Eg:- Covalency of chlorine is one, oxygen is two, and that of nitrogen is three.
DOUBLE BONDS IN CARBON DIOXIDE MOLECULE
FORMAL CHARGE
In the case of polyatomic ions, the net charge is possessed by the ion as a whole and
not by a particular atom.
The formal charge is defined as the difference between the number of valence
electrons of that atom in the free State and the number of electrons assigned to that
atom in the Lewis Structure.
Formal Charge = [Total No. of valence electrons in the free atom] ― [Total No. of lone
pair of electrons] ― ½ [Total No. of bonding electrons]
Eg:- Consider ozone molecule
The Lewis structure of ozone may be drawn as
The formal charge on
PREPARED BY HAIZEL G. ROY, HSST (HG) CHEMISTRY, GOVT. H.S.S. KALAMASSERY, ERNAKULAM Page | 3
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