A slide on Valence bond theory

1. VALENCE BOND THEORY
• In valence-bond theory, covalent bonding is
explained on the basis of the concentrationof
electron density between two nuclei.
• This is the consequence of the sharing of
space or overlap of the valence atomic orbital
of one atom with the valence atomic orbital of
another atom.
• Covalent bond is formed by overlap of atomic
orbitals. 1

2. • The overlap can occur with different
symmetries, 2 main symmetries for use are:
(a) σ = sigma symmetry
(b) π = pi symmetry
2

3. σ = sigma symmetry
• The sigma bond results in an increase in
electron density symmetrical about the
internuclear axis, z-axis.
s s σ
3
z

4. • Sigma bonds can also be formed when 2pz
orbitals interact
S pz σ
4

5. • Sigma bonds can also be formed when 2pz
orbitals interact
pz pz σ
• S + dz2 = σ
• S + dx2-y2 = σ (polyatomic molecule) 5

6. π = pi symmetry
• In the formation of pi-bond, the overlap of
atomic orbitals results in the internuclear axiss
lying on a NODAL PLANE. There is increased
electron density above and below the
internuclear axis.
• Pi-bonds are usually formed after sigma bond
is formed.
6

7. • Py py π
px + px= π
px+ dxy = π
px + dxz = π
py + dxy = π
7

8. THE CONCEPT OF HYBRIDIZATION
• The atomic orbitals (Aos) that combine can
led to the original atomic orbital present in
the ground state of the atom.
• However, if the geometry of these original Aos
is such that effective overlap cannot occur in
the known geometry of the molecule
• The AO of an atom can re-configure into a
different configuration. 8

9. • These re-configured orbitals are said to be
hybridized.
• Hybridization is the mixing of atomic orbitals
of different energy to obtain orbitals of
equivalent amount of energy.
• The orbitals so obtained are referred to as
hybrid orbitals.
9

10. • Hybridization gives a set of Aos that can
overlap more effectively with Aos of other
atoms in the molecule.
• This yields stronger bonds and molecules with
lower energies.
• This concept (hybridization)is very important
in the explanationof molecular shapes
10

11. • In applying the concept of hybridization, the ff points
must be noted.
❖ Hybridized orbitals can only form sigma bonds.
❖ Not all orbitals on an atom need to be hybridized, only
orbitals required for bonding are hybridized.
❖Hybridized orbitals are all half full
❖Hybridized orbitals on an atom overlap with either
hybridized or normal orbital on another atom or with
both depending on the moleculer geometry.
11

12. • This type of hybridizationinvolves the mixing
of an s atomic orbital with one p atomic
orbital to form two hybrid orbitals, designated
as sp.
• This type of hybrization can be explained by
looking at the covalent bonding in BeCl2.
sp Hybridization
12

13. • The ground state electron configuration of Cl is [Ne](3s23p5)
• There is an unpaired electron in the 3p orbital which can be
paired with an unpaired Be electron.
• The concept of hybridization will help in understanding the
identity of the orbitals in Be which overlap with those on the
Cl atoms to form the Be-Cl bonds.
13

14. The Be atom now has two unpaired electrons and can therefore form two polar covalent
bonds with Cl atoms.
The resulting orbitals are neither an s nor a p orbital, but hybrid orbitals called sp
hybrid orbital.
The orbital diagram for sp hybrid orbital
14

15. • The two new orbitals are identical in shape, but their large
lobes point in opposite directions.
• The fact that the two sp hybrid orbitals are equivalent but point
in opposite directions, allows the formation of bond with the
two chlorine atoms in BeCl2.
• The two sp hybrid orbitals lie in the same plane, 180º apart
from one another.
• This gives BeCl2 molecule a linear shape
• The remaining two 2p atomic orbitals of Be remain
unhybridized and are vacant.
15

16. sp2 Hybridization
• The sp2 hybridization involves the mixing of one s
atomic orbital with two p atomic orbitals.
• sp2 hybridization gives three hybrid orbitals.
• E.g. BCl3, mixing the 2s and two of the 2p atomic
orbitals of Boron yields three equivalent sp2 hybrid
orbitals.
• The three sp2 hybrid orbitals lie in the same plane,
120º apart from one another. This gives BCl3
molecule a trigonal-planar shape
•
16

17. • The ground state electron configuration of Cl is [Ne](3s23p5)
▪ Ground and excited states of boron
▪ Formation of sp2 hybrid orbitals
17

18. sp3 Hybridization
• sp3 hybridization is derived from the mixing of one s
atomic orbital with three p atomic
• sp3 hybridization gives four hybrid orbitals.
• E.g. CH4, mixing the 2s and three of the 2p atomic
orbitals of carbon yields four equivalent sp3 hybrid
orbitals.
• Each sp3 hybrid orbital has a large lobe that points
toward one vertex of a tetrahedron at 109.5º apart from
one another. This gives CH4 molecule a tetrahedral
18

19. • The ground state electron configuration of H 1s1
▪ Ground and excited states of carbon
▪ Formation of sp3 hybrid orbitals
19