Please find the V-Quiz DPP
link in the description box
Valence Bond Theory
👉 To form a covalent bond overlapping occurs
between half filled valence shell orbitals of the two atoms
👉 Resulting bond acquires a pair of electrons with
opposite spins to get stability
👉 Orbitals come closer to each other from the direction
in which there is maximum overlapping
👉 So covalent bond has directional character.
👉 Extent of overlapping ∝ strength of chemical bond.
Valence Bond Theory
👉 Extent of overlapping depend on two factors.
❏ Nature of orbitals
p, d and f are directional orbitals ⟶ more overlapping
s-orbitals ⟶ non directional - less overlapping
❏ Nature of overlapping
Co-axial overlapping - extent of overlapping more
Collateral overlapping - extent of overlapping les
Order of strength of Co-axial overlapping -
p-p>s-p>s-s
Overlapping of Atomic Orbitals
When two atoms come close to each other, there is
overlapping of atomic orbitals
Positive overlap Zero overlap
Negative overlap
Positive or in phase overlapping
● When the phase of two interacting orbital is same
Negative or out phase overlapping
● When the phase of two interacting orbital is opposite
Zero Overlapping
● When the orientation of two interacting atomic orbital is
such that there is no overlapping of the orbital
Type of Overlap & Bond
Sigma bonds
📖 Sigma Bond
Formed by end to end overlap (head-on) of bonding orbitals
along the Internuclear axis
Also called head on overlap or axial overlap
Valence Bond Theory
s-s overlapping
Example:
s-p overlapping
Example:
p-p overlapping
Example:
pi bonds
This type of covalent bond is formed by the side wise (or
lateral) overlapping of half filled atomic orbitals of bonding
atoms.
The strength of bonds formed by 2s - 2s, 2p - 2p and 2p- 2s overlap
has the order:
A s-s>p-p>p-s
B s-s>p-s>p-p
C p-p>p-s>s-s
D p-p>s-s>p-s
The strength of bonds formed by 2s - 2s, 2p - 2p and 2p- 2s overlap
has the order:
A s-s>p-p>p-s
B s-s>p-s>p-p
C p-p>p-s>s-s
D p-p>s-s>p-s
Solution:
Extent of overlapping decreases in the order
p-p>p-s>s-s
Which of the following overlaps is incorrect [assuming z-axis to be
the internuclear axis] ?
A ‘a’ & ‘b’
B ‘b’ & ‘d’
C only ‘d’
D None of these
Which of the following overlaps is incorrect [assuming z-axis to be
the internuclear axis] ?
A ‘a’ & ‘b’
B ‘b’ & ‘d’
C only ‘d’
D None of these
Solution:
Draw the type of overlaps between
(a) s and px (b) px and px (c) py and py
(d) pz and pz
(e) (f)
(g) s and dyz (h)
If internuclear axis is z-axis. Identify them as σ, π, δ bond wherever bond is
formed
Solution:
Molecular orbital Theory
Postulates of MOT
☛ The electrons in molecules are present in various molecular
orbital as the electron of atom are present in various atomic
orbital.
☛ The atomic orbitals of comparable energies and proper
symmetry combine to form molecular orbital.
☛ An electron in an atomic orbital is influenced by one nucleus,
but in molecular orbital it is influenced by two or more nuclei.
Postulates of MOT
☛ The number of molecular orbital formed is equal to the
number of combining atomic orbitals. When two atomic
orbitals combine, two molecular orbitals are formed. One
is known as bonding molecular orbital while the other is
called antibonding molecular orbital.
Postulates of MOT
☛ The bonding molecular orbital has lower energy and
hence greater stability than the corresponding
antibonding molecular orbital.
☛ Just as the electron probability distribution around a
nucleus in an atom is given by an atomic orbital, the
electron probability distribution around a group of nuclei
in a molecule is given by a molecular orbital.
Postulates of MOT
☛ The molecular orbitals like atomic orbitals are filled
in accordance with the Aufbau principle, obeying the
Pauli’s exclusion principle and the Hund’s
Linear Combination of Atomic Orbitals (LCAO)
Bonding molecular orbital (BMO) Antibonding molecular orbital (ABMO)
Bonding MO is the result of the linear ABMO is result of linear combination of
combination of AO when their wave AO when their wave function are
function are added - subtracted -
𝝍b = 𝝍A+ 𝝍B 𝝍a = 𝝍A – 𝝍B
It does not have node. It always have a node between two
nuclei bonded atom.
Charge density increases between Charge density decreases in between
two nuclei resulting in attraction two nuclei, leads to repulsion between
between two atoms two atoms.
Energy of BMO is less, hence stable. Energy of ABMO is high, hence unstable.
Energy level diagram for molecular orbitals
Two 1s atomic orbitals combine to form two molecular orbitals :
Bonding MO: Antibonding MO: σ*1s
σ1s
2s and 2p atomic orbitals give rise to the following eight
molecular orbitals:
Bonding MO: Antibonding MO:
σ2s σ2pz σ2px σ2py σ∗ 2s
σ∗ 2pz π∗ 2px π∗ 2py
Molecular orbital Theory
For B2, C2, N2 the increasing order of energies of various
Molecular orbital is:
σ1s < σ* 1s < σ2s< σ* 2s < (π2px= π2py ) < σ2pz < (π*2px=π* 2py ) < σ* 2pz
Molecular orbital Theory
The increasing order of energies of various molecular orbitals for O 2
and F2 is :
σ1s < σ* 1s < σ2s< σ* 2s< σ2pz < (π2px= π2py ) < (π*2px=π* 2py ) < σ* 2pz
Molecular orbital theory {Less than 14 electron}
Molecular orbital theory {More than 14 electron}
What information can be obtained from the MOT Theory
❏ Bond Order
❏ Stability of Molecules ❏ Nature of Bond
❏ Bond Length
❏ Magnetic Nature
Bond order
Nb= number of electrons in bonding orbitals
Na= number of electrons in antibonding orbitals
Nature of the bond
If Bond Order = 1, Single Covalent
Bond
e.g. H2, F2 Molecule
If Bond Order = 2,
Double Covalent Bond
e.g. O2 Molecule
If Bond Order = 3, Triple Covalent
Bond
e.g. N2 Molecule
Stability of Molecules
● Stability of the molecule, on the whole, depends on
these factors:
● These are :
a. If Nb > Na , the molecule is stable.
b. If Nb < Na, the molecule is unstable.
Bond Length
Bond length is inversely proportional to bond order
☛ More is the bond order, lesser is the bond length
Magnetic Nature
☛ If all the molecular orbitals in a molecule are doubly
occupied the substance is DIAMAGNETIC.
☛ If one or more molecular orbitals are singly occupied, the
substance is PARAMAGNETIC.
Correct order of bond energy of C–O bond is :
A CO32– > CO2 > CO
B CO2 > CO > CO32–
C CO > CO2 > CO32–
D None of these.
Correct order of bond energy of C–O bond is :
A CO32– > CO2 > CO
B CO2 > CO > CO32–
C CO > CO2 > CO32–
D None of these.
Solution:
Among the following species, which has the minimum bond
length?
A B2
B C2
C F2
D
Among the following species, which has the minimum bond
length?
A B2
B C2
C F2
D
Solution:
The following molecules / species have been arranged in the
order of their increasing bond orders. Identify the correct
order.
A III < II < I < IV
B IV < III < II < I
C III < II < IV < I
D II < III < I < IV
The following molecules / species have been arranged in the
order of their increasing bond orders. Identify the correct
order.
A III < II < I < IV
B IV < III < II < I
C III < II < IV < I
D II < III < I < IV
Solution:
Which of the following best describes the diagram below of a
molecular orbital?
A A non-bonding orbital
B An antibonding σ orbital
C A bonding π orbital
D An antibonding π orbital
Which of the following best describes the diagram below of a
molecular orbital?
A A non-bonding orbital
B An antibonding σ orbital
C A bonding π orbital
D An antibonding π orbital
Solution:
You can contact me
Beastchemist
Beastchemist
@BEASTCHEMIST
Mohit Ryan Sir
Join Vedantu JEE
Telegram
channel NOW!
Assignments
Notes
Daily Update
https://vdnt.in/JEEVedantu
