Chemical Bonding: A Molecule
Chemical Bonding: A Molecule
Chemical Bonding: A Molecule
• Kössel-Lewis approach
• Valence Shell Electron Pair Repulsion (VSEPR)
Theory
• Valence Bond (VB) Theory
• Molecular Orbital (MO) Theory.
Kossel-lewis
Approach
Lewis postulated that atoms achieve the stable octet when
they are linked by chemical bonds.
Na Na+ + e-
Cl + e- Cl-
When two atoms share one electron pair they are said to be
joined by a single covalent bond
Double Bond
=
Consider the ozone molecule O3
Hence,
O3 molecule along with the formal charges is
Limitations of the Octet Rule
Odd-electron molecules:
R= rA + rB
Where R is the bond length
RA and RB are the covalent
Covalent
Radius
The covalent radius is measured approximately as the radius of
an atom’s core which is in contact with the core of an adjacent
atom in a bonded situation.
Vander waals radius of oxygen
Similarly,
O2(g) O(g) +O(g) ; ∆aH- = 498 kJ mol-1
N2(g) N(g) + N(g) ; ∆aH- = 946 kJ mol-1
HCl(g) H(g) + Cl(g) ; ∆aH- = 431 kJ mol-1
Bond Order
• The number of bonds between the two atoms in a molecule
is given as Bond Order
• The bond order in H2 (single shared electron pair) is
1
• The bond order in O2 (two shared electron pairs) is 2
The Valence Shell Electron Pair
Repulsion (VSEPR) Theory
Main postulates of VSEPR theory are:
• The shape of a molecule depends upon the number of
valence shell electron pairs (bonded or unbonded) around the
central atom.
• Pairs of electrons in the valence shell repel one another since
their electron are negatively charged.
• These pairs of electrons tend to occupy such positions in
space that minimize repulsion and thus maximize distance
between them.
• The valence shell is taken as a sphere with the electron pairs
localizing on the spherical surface at maximum distance from
one another.
• A multiple bond is treated as if it is a single electron pair and
the two or three electron pairs of a multiple bond are treated
as a single super pair.
• Where two or more resonance structures can represent a
molecule, the VSEPR model is applicable to any such
structure
The repulsive interaction of electron pairs decrease in the order:
• When the two atoms are at large distance from each other, there is no
interaction between them. As these two atoms approach each other,
there is attractive and repulsive forces.
Consider two hydrogen atoms A and B approaching each other having
nuclei NA and NB electrons present in them are represented by eA and
eB
• Nucleus of one atom and its own electron that is NA – eA and NB - eB
When two hydrogen atoms are near then their atomic orbitals
undergo partial interpenetration. This partial merging of atomic
orbitals is called Overlapping of Atomic Orbitals
The covalent bond is classified into two types depending upon the
types of overlapping. They are
• Sigma(σ) Bond
• Pi(π) Bond
Sigma(σ)
Bond
This bond is formed by the end to end (head-on) overlap of
bonding orbitals along the internuclear axis. This is called as head
on overlap or axial overlap.
• S-S Overlapping: Overlap of two half filled s-
orbitals
BeCl2
C2H2
sp2 Hybridisation
The mixing of one s and two p-orbitals in order to form three
equivalent sp2 hybridised orbitals
Examples
BF3
C2H4
sp3 Hybridisation
The mixing of one s-orbital and three p-orbitals of the valence
shell to form four sp3 hybrid orbitals
sp3 Hybridisation in C2H6
molecule:
• In ethane molecule both the carbon atoms assume sp3 hybrid
state.
• One of the four sp3 hybrid orbitals of carbon atom overlaps
axially with similar orbitals of other atom to form sp3 - sp3
sigma bond while the other three hybrid orbitals of each
carbon atom are used in forming sp3–s sigma bonds with
hydrogen atoms
C2H6
Hybridisation of Elements involving d-Orbitals
• σ(sigma)
• π(pi)
• δ (delta)
1s atomic orbitals
2PZ atomic orbitals
2PX atomic orbitals
Stability of
Molecules:
• If Nb > Na then it is a stable molecule
Lithium molecule
In Lithium molecule there are six electrons.
The electronic configuration of Lithium molecule is
Li2: (σ1s)2(σ*1s)2(σ2s)2
Bond order of Lithium molecule is (4 - 2)/2 = 1
Lithium molecule is stable and since it has no unpaired electrons
Carbon molecule
There are twelve electrons in carbon molecule
Electronic configuration of Carbon molecule is:
C2: (σ1s)2(σ*1s)2(σ2s)2(σ*2s)2(∏2Px2 -∏2Py2)
The bond order of Carbon molecule is (8 - 4)/2 = 2
Carbon molecule is
diamagnetic
Oxygen molecule
In Oxygen molecule there are 16 electrons i.e., each oxygen
atom has 6 electrons
Electronic configuration of Oxygen molecule is
O2: (σ1s)2(σ*1s)2(σ2s)2(σ*2s)2(σ1Pz)(∏2Px2 -∏2Py2)(∏*2Px1 -
∏*2Py1)
Bond order of oxygen molecule is (10-6)/2 = 2
Oxygen molecule is
paramagnetic
Hydrogen
Nitrogen, Bonding
oxygen and fluorine are the highly electronegative
elements. When they are attached to a hydrogen atom to form
covalent bond, the electrons of the covalent bond are shifted
towards the more electronegative atom.