CHEMICAL BONDING AND MOLECULAR STRUCTURE 6 Flashcards
explain sp hybridisation
This type of hybridisation
involves the mixing of one s and one p orbital
resulting in the formation of two equivalent
sp hybrid orbitals. The suitable orbitals for
sp hybridisation are s and pz
, if the hybrid
orbitals are to lie along the z-axis. Each sp
hybrid orbitals has 50% s-character and
50% p-character. Such a molecule in which
the central atom is sp-hybridised and linked
directly to two other central atoms possesses
linear geometry. This type of hybridisation is
also known as diagonal hybridisation.
The two sp hybrids point in the opposite
direction along the z-axis with projecting
positive lobes and very small negative lobes,
which provides more effective overlapping
resulting in the formation of stronger bonds.
explain sp2 hybridisation
: In this hybridisation
there is involvement of one s and two
p-orbitals in order to form three equivalent
sp2 hybridised orbitals.There is 33.3% s character and 66.7% p charcater.
explain sp3 hybridisation
there is
mixing of one s-orbital and three p-orbitals
of the valence shell to form four sp3 hybrid
orbital of equivalent energies and shape. There
is 25% s-character and 75% p-character in
each sp3 hybrid orbital. The four sp3 hybrid
orbitals so formed are directed towards the
four corners of the tetrahedron.
Hybridisation of Elements
involving d Orbitals
The elements present in the third period
contain d orbitals in addition to s and p
orbitals. The energy of the 3d orbitals are
comparable to the energy of the 3s and 3p
orbitals. The energy of 3d orbitals are also
comparable to those of 4s and 4p orbitals.
As a consequence the hybridisation involving
either 3s, 3p and 3d or 3d, 4s and 4p is
possible. However, since the difference in
energies of 3p and 4s orbitals is significant, no
hybridisation involving 3p, 3d and 4s orbitals
is possible.
why is pcl5 extremely reactive
In PCl5 the five sp3d orbitals of
phosphorus overlap with the singly occupied
p orbitals of chlorine atoms to form five P–Cl
sigma bonds. Three P–Cl bond lie in one
plane and make an angle of 120° with each
other; these bonds are termed as equatorial
bonds. The remaining two P–Cl bonds–one
lying above and the other lying below the
equatorial plane, make an angle of 90° with
the plane. These bonds are called axial bonds.
As the axial bond pairs suffer more repulsive
interaction from the equatorial bond pairs,
therefore axial bonds have been found to
be slightly longer and hence slightly weaker
than the equatorial bonds; which makes PCl5
molecule more reactive.
who gave molecular orbital theory?
givenmy F Hund and RS Mulliken in 1932.
what are the postulates of molecularorbital theory?
(i) The electrons in a molecule are present in molecular orbitals
(ii) The atomic orbitals of comparable energies and proper symmetry combine t fomr molecularorbitals.
(iii) The elctrons in molecular orbitals are influenced by two or more nuclei depedning on the no of atoms present in the moelcule. Hence molecular orbitals are polycentric.
(iv)The number of molecular orbitals is equal to the no of combining atomic orbitals.
(v) If 2 atoms orbitals combine, 2 molecular orbitals are formed, one is called the bonging molecular orbital and the other one is callled as antibonding molecular orbital.
(vi) The bonding molecular orbitals has lessser energy and hence is considered to be more stable, while antibonding molecular orbitals are less stable and more energetic,
(vii)The electron probability distrubutionin a molecue is given my molecular orbitals.
(viii) Orbitals are filled in accordance to aufbau principle, Pauli principle and Hund’s rule.
What is linear combination of atomic orbitals or LCAO
the formation of
molecular orbitals may be described by the
linear combination of atomic orbitals that can
take place by addition and by subtraction of
wave functions of individual atomic orbitals
as shown below :
ψMO = ψA + ψB
Therefore, the two molecular orbitals
σ and σ* are formed as :
σ = ψA + ψB
σ* = ψA – ψB
The molecular orbital σ formed by the
addition of atomic orbitals is called the bonding
molecular orbital while the molecular orbital
σ* formed by the subtraction of atomic orbital
is called antibonding molecular orbital
explain the formation of molecular orbitals with respect to interference
Qualitatively, the formation of molecular
orbitals can be understood in terms of the
constructive or destructive interference of the
electron waves of the combining atoms. In the
formation of bonding molecular orbital, the
two electron waves of the bonding atoms
reinforce each other due to constructive
interference while in the formation of antibonding molecular orbital, the electron
waves cancel each other due to destructive
interference.
explain the e- density in molecular orbitals
As a result, the electron density in
a bonding molecular orbital is located between
the nuclei of the bonded atoms because of
which the repulsion between the nuclei is very
less while in case of an antibonding molecular
orbital, most of the electron density is located
away from the space between the nuclei.
Infact, there is a nodal plane (on which the
electron density is zero) between the nuclei
and hence the repulsion between the nuclei is
high.
explain the energies of the molecular oritals
Electrons placed in a bonding molecular
orbital tend to hold the nuclei together and
stabilise a molecule. Therefore, a bonding
molecular orbital always possesses lower
energy than either of the atomic orbitals that
have combined to form it. In contrast, the
electrons placed in the antibonding molecular
orbital destabilise the molecule. This is
because the mutual repulsion of the electrons
in this orbital is more than the attraction
between the electrons and the nuclei, which
causes a net increase in energy.
what is the net sum of energies of molecular orbitla s
It may be noted that the energy of the
antibonding orbital is raised above the
energy of the parent atomic orbitals that
have combined and the energy of the bonding
orbital has been lowered than the parent
orbitals. The total energy of two molecular
orbitals, however, remains the same as that
of two original atomic orbitals.
Conditions for the Combination of
Atomic Orbitals
- The combining atomic orbitals must
have the same or nearly the same energy.
This means that 1s orbital can combine with
another 1s orbital but not with 2s orbital
because the energy of 2s orbital is appreciably
higher than that of 1s orbital. This is not true
if the atoms are very different. - The combining atomic orbitals must
have the same symmetry about the
molecular axis. By convention z-axis is takenas the molecular axis. It is important to note
that atomic orbitals having same or nearly
the same energy will not combine if they do
not have the same symmetry. For example,
2pz
orbital of one atom can combine with 2pz
orbital of the other atom but not with the
2px
or 2py orbitals because of their different
symmetries - The combining atomic orbitals must
overlap to the maximum extent.
Greater
the extent of overlap, the greater will be the
electron-density between the nuclei of a
molecular orbital.
