Notes

Chemical Bond
It is defined as the attractive forces which hold the various chemical constituents (atoms, ions, etc.) together in different chemical species.
Bond forms to get the stability. with a release of energy.
Kossel-Lewis Approach to Chemical Bonding
According to this theory. atoms take part in the bond formation to complete their octet or to acquire the electronic configuration of the nearest inert gas atoms (Octet rule). This can be achieved by gaining, losing or sharing the electrons.
Lewis Symbols
Valence electrons are reported by dots around the chemical symbol of element, e.g.,
Ionic Bond
A chemical bond formed by complete transference of electrons from one atom (metal) to another (non-metal) and hence, each atom acquire the stable nearest noble gas configuration, is called ionic bond or electrovalent bond, e.g., formation of sodium chloride
Favourable factors for the formation of ionic bonds
(i) Metal should have lowest ionisation enthalpy.
(ii) Non-metal must have highest electron gain enthalpy.
(iii) The energy released during the formation of 1 mole of crystal lattice, i.e., lattice enthalpy must be high.
[Some elements exhibit variable electrovalency. The reason for this iS unstable configuration of penultimate orbit and inert pair effect].
Ions
Species carrying either positive or negative charge are termed as ions. Species carrying positive charge are called cations and that carrying negative charge are called anions. Metals usually form cation while non-metals (except H) usually form anions.
General Characteristics of Ionic Compounds
1. Ionic compounds are usually solids in nature.
2. Ionic compounds have high melting and boiling points.
3. Ionic compounds are soluble in polar solvents like water but insoluble in non-polar solvents like benzene, C014 etc.
4. Ionic compounds are good conductor in molten state and in aqueous solution.
5. Ionic compounds has crystal structure.
Born Haber Cycle
This cycle is based upon the fact that the formation of an ionic compound may occur either by direct combination of the elements or by an alternate process in which :
1. The reactants (metal) are vaporised to convert into gaseous state.
2. The gaseous atoms are converted into ion.
3. The gaseous ions are combined to form ionic lattice of molecule. e.g., formation of NaCI can be shown as
where, S = enthalpy of sublimation
I = ionisation energy
D = dissociation energy
E = electron affinity
U = lattice energy
Q = total enthalpy change
Method of Writing Formula of Ionic Compound
1. Write the symbol of cation at the left and anion at the right.
2. Write their electrovalencies in figures on the top of each symbol as AXBY.
3. Divide their valencies by HCF.
4. Now apply criss-cross rule as
i.e., formula is A B .
e.g., formula of aluminum sulphate
is Al (So ) .
Covalent Bond
A chemical bond formed between two atoms by mutual sharing of electrons between them so as to complete their octets or duplets, is known as covalent bond and the number of electrons contributed by each atom is known as covalency. e.g., formation of CI /
In covalent bonding, the shared pairs of electrons present between the atoms are called bond pairs while unshared pairs or non-bonding electron pairs are known as lone pairs.
Polar Covalent Bond
If a covalent bond is formed between the different ~toms, the shared pair is displaced towards the more electronegative atom causing greater concentration of electron density around the more electronegative atom. Such a covalent bond develops some ionic character and is called polar covalent bond (e.g., H-CI).
Properties of Covalent Compounds
1. In general, covalent compounds exist in the liquid or gaseous state at room temperature due to magnitude of intermolecular forces.
2. Covalent compounds have low melting and boiling points.
3. Covalent compounds are generally poor conductors of electricity because they do not contain free electrons or ions to conduct electricity.
4. They are soluble in non-polar solvents like