Chemical Bonding
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INTRODUCTION
It is well known fact that except inert gases, no other element exists as independent atom under ordinary
conditions.
Most of the elements exist as molecules which are cluster of atoms. How do atoms combine to form
molecules and why do atoms form bonds? Such doubts will be discussed in this chapter.
A molecule will only be formed if it is more stable and has a lower energy, than the individual atoms.
Chemical Bond
A force that acts between two or more atoms to hold them together as a stable molecule.
It is union of two or more atoms involving redistribution of e– among them.
This process accompanied by decrease in energy.
Decrease in potential energy (P.E.) a Strength of the bond.
Therefore molecules are more stable than atoms.
Tendency to acquire minimum energy
(a) When two atoms approaches to each other. Nucleus of one atom attracts the electron of another
atom.
(b) Two nuclei and electrons of both the atoms repells each other.
(c) If net result is attraction, the total energy of the system (molecule) decreases and a chemical bond
forms.
(d) So Attraction a 1/energy is directly propertional to stability
(e) Bond formation is an exothermic process
Tendency to acquire noble gas configuration :
(a) Atom combines to acquire noble gas configuration.
(b) Only outermost electrons i.e. ns, np and (n-1)d shells electrons participate in bond formation.
(c) Inert gas elements do not participate in bond formation, as they have stable electronic configuration
hence will have minimum energy. (Stable configuration 1s2 or ns2np6)
KOSSEL - LEWIS APPROACH TO CHEMICAL BONDING
Every atom has a tendency to complete its octet in outermostshell
H has the tendency to complete its duplet.
To acquire inert gas configuration atoms loose or gain electron or share electron.
The tendency of atoms to achieve eight electrons in their outer most shell is known as Lewis octet rule
Exception of Octet Rule
(a) Incomplete octet molecules :- or (electron defficient molecules)
Compound in which octet is not complete in outer most orbit of central atom.
Example - Halides of IIIA groups, BF3, AlCl3, BCl3, hydride of III A/13th group etc
b) Expansion of octet or (electron efficient molecules)
Compound in which central atom has more than 8e– in outermost orbits.
Example - In PCl5, SF6 and IF7 the central atom P, S and I contain 10, 12,
and 14 electrons respectively.
(c) Pseudo inert gas configuration :-
Cations which contain 18 electrons in outermost orbit
Ex. Ga+3, Cu+, Ag+, Zn+2, Cd+2, Sn+4, Pb+4 etc.
Electronic configuration of Ga - 1s2,2s22p6,3s23p63d10,4s24p1
Electronic configuration of Ga+3-1s2.2s2,2p2
d) Cations having electron between 9 to 17 in their outer most shell
Ex. Mn+2, Fe+2, Fe+3, Ti+2 etc.
Electronic configuration of Fe - 1s2,2s22p6,3s23p63d6,4s2
(e) Odd electron molecules :-
Central atom have an unpaired electron or odd no (7e–, 11e– etc) of electrons in their outer most shell.
e.g. NO, NO2, ClO2 etc
It is defined as the combining capacity of the elements. The word valency is derived from an Italian word
"Valentia" which means combining capacity.
Old concept : Given by : Frankland
Valency with respect to Hydrogen : Valency of H = 1
It is defined as the number of hydrogen attached with a particular element
IA IIA IIIA IVA VA VIA VIIA
NaH MgH2 AlH3 SiH4 PH3 H2S HCl
Valency 1 2 3 4 3 2 1
Note : Valency w.r.t. H across the period increases upto 4 and then again decreases to 1.
Valency with respect to oxygen : Valency of 'O' = 2
It is defined as twice the number of oxygen atoms attached with a particular atom.
IA IIA IIIA IVA VA VIA VIIA
Na2O MgO Al2O3 SiO2 P2O5 SO3 Cl2O7
Valency 1 2 3 4 5 6 7
Note : Valency with respect to oxygen increases from 1 to 7 across the period. Valency w.r.t. 'O' is equal to the
group number.
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