Concept Of Hybridization
While
formation of simple molecules could be explained adequately by overlap of
atomic orbitals, the formation of molecules of Be, B and C present problems of
greater magnitude having no solution with the previous theory. To explain fully
the tendency of these atoms to form bonds and the shape or geometry of their
molecules, a new concept called Hybridization is introduced.
Atomic orbitals and Hybrid orbitals
❒According to
this concept, we may mix any number of atomic orbitals of an atom, which differ
in energy only slightly, to form new orbitals called Hybrid orbitals. The
mixing orbitals generally belong to the same energy level (say 2s and 2p orbitals
may hybridize).
❒The total number
of hybrid orbitals formed after mixing, is invariably equal to the number of
atomic orbitals mixed or hybridized.
❒An important
characteristic of hybrid orbitals is that:
(1)
they are all identical in respect of energy and directional character.
(2) They,
however, differ from the original atomic orbitals in these respects.
(3) They
may also differ from one another in respect of their arrangement in space.
i.e., orientation.
(4) Like
pure atomic orbitals, the hybrid orbitals of an atom shall have a maximum of two
electrons with opposite spin.
(5) Hybrid
orbitals of an atom may overlap with other bonding orbitals (pure atomic or
hybrid) on other atoms or form molecular orbitals and hence new bonds.
❒Hybridization
is defined as : the phenomenon of mixing up (or merging) of orbitals of an atom
of nearly equal energy, giving rise to entirely new orbitals equal in number to
the mixing orbitals and having same energy contents and identical shapes.
Rules of Hybridization
For hybridization to occur, it is
necessary for the atom to satisfy the following conditions :
(1)
Orbitals on a single atom only would undergo hybridization.
(2)
There should be very little difference of energy level between the orbitals
mixing to form hybrid orbitals.
(3)
Number of hybrid orbitals generated is equal to the number of hybridizing
orbitals.
(4) The
hybrid orbitals assume the direction of the dominating orbitals.
For example, if s and p orbitals are
to hybridize, the s orbital having no directional character, does not
contribute towards the direction when p orbitals determine the directional
character of the hybrid orbitals.
(5) It
is the orbitals that undergo hybridization and not the electrons.
For example, four orbitals of an
oxygen atom (2 2 , 2 2 , 2 1 , 2 1 ) s px py pz belonging to second level
(i.e., 2s, 2px, 2py, 2pz) can hybridize to give four hybrid orbitals, two of
which have two electrons each (as before) and the other two have one electron
each.
(6) The
electron waves in hybrid orbitals repel each other and thus tend to be farthest
apart.
Types of Hybridization
❒Since
hybridization lends an entirely new shape and orientation to the valence
orbitals of an atom, it holds a significant importance in determining the shape
and geometry of the molecules formed from such orbitals.
❒Depending upon
the number and nature of the orbitals undergoing by hybridization, we have
various types of hybrid orbitals. For instance s, p, and d orbitals of simple atoms
may hybridize in the following manner:
(1) sp Hybridization
(2) sp2 Hybridization
(3) sp3 Hybridization
(4) Hybridization involving (d)
orbitals
(1) sp Hybridization
❒The mixing of
an s and a p orbital only leads to two hybrid orbitals known as sp hybrid
orbitals after the name of an s and a p orbital involved in the process of
hybridization. The process is called sp hybridization.
❒In sp
hybridization process, Each sp orbital has 50%, s-character and 50% p-character.
❒Orbitals thus generated
are the seat of electrons which have a tendency to repel and be farther apart.
In order to do so the new orbitals arrange themselves along a line and are, therefore,
often referred to as Linear hybrid orbitals. This gives an angle of 180º
between the axes of the two orbitals.
❒It is clear
from the following Figure that an sp orbital has two lobes (a character of p
orbital) one of which is farther than the corresponding s or p orbitals and
also protrudes farther along the axis. It is this bigger lobe that involves
itself in the process of an overlap with orbitals of other atoms to form bonds.
It will be seen later on that the smaller lobes of hybrid orbitals are neglected
while considering bond formation.
❒Examples : BeF2,
BeCl2, etc.
(2) sp2 Hybridization
❒When an s and
two p orbitals mix up to hybridize, there result three new orbitals called sp2
hybrid orbitals (spoken as ‘sp two’).
❒ In sp2
hybridization process, Each sp2 hybrid orbital has 33% s-character and
67% p-character.
❒As the three
orbitals undergoing hybridization lie in a plane, so do the new orbitals. They
have to lie farthest apart in a plane which can happen if they are directed at
an angle 120º to one another as shown in Fig. (b).
❒It is for this
reason that sp2 hybrid orbitals are also called Trigonal hybrids,
the process being referred to as Trigonal hybridization.
❒The sp2
hybrid orbitals resemble in shape with that of sp hybrid orbitals but are
slightly fatter.
❒Examples : BF3,
NO3–, etc.
(3) sp3 Hybridization
❒The four new
orbitals formed by mixing an s and three p orbitals of an atom are known as sp3
hybrid orbitals.
❒In sp3
hybridization process, Each sp3 hybrid orbital has 25% s-character and
75% p-character. Since mixing of orbitals takes place in space, the four hybrid
orbital would also lie in space.
❒An arrangement in
space that keeps them farthest apart is that of a tetrahedron. Thus each of the
four hybrid orbitals are directed towards the four corners of a regular tetrahedron
as shown in Fig (b).
❒Because of
their tertrahedral disposition, this type of hybridization is also called
Tetrahedral hybridization.
❒They are of the
same shape as that of the previous two types but bigger in size. They are
disposed in manner such that the angle between them is 109.5º as shown in the
following Figure:
Examples : CH4 , SO42-
, ClO4- , etc.
(4) Hybridization involving (d) orbitals
❒There are
several types of hybridization involving d orbitals. Since the d orbitals have
a relatively complex shape, we will consider here only some of the common
types.
❒ The most important
of these are sp3d hybridization, sp3d2
hybridization and sp2d hybridization.
sp3d
❒In sp3d
hybridization the orbitals involved are one of s type, three of p type and one
of d type.
❒The five new
orbitals will be farthest apart by arranging three of them in a plane at an
angle of 120º to one another and the other two in a direction perpendicular to
the plane.
❒The figure obtained
by joining the ends assumes the shape of a trigonal bipyramid.This type of hybridization
is, therefore, called Trigonal bipyramidal hybridization.
sp3d2
❒When two (d)
type of orbitals take part in hybridization with one s type and three p type orbitals,
six hybrid orbitals called sp3d2 hybrid orbitals are
created.
❒To be away from
one another four of them are dispersed in a plane at an angle of 90º each and
the rest two are directed up and below this plane in a direction perpendicular
to it. On joining their corners, an octahedron results and this type of hybridization
also gets the name Octahedral hybridization
sp2d
❒So far we have
been considering the hybridization of orbitals belonging to the same energy level
(say 3s, 3p, and 3d orbitals) of an atom. But this may not necessarily be so
always. In fact, there is very little energy difference between 3d, 4s and 4p
orbitals which may undergo sp2d hybridization.
❒The d orbital
involved in this type of hybridization has the same planar character as the two
p orbitals have and the hybrid orbitals will also be planar, dispersed in such
a way so as to be farthest apart i.e., subtending an angle of 90º between them.
This gives a square planar arrangement for them and the hybridization is,
therefore, called Square planar hybridization.
❒The directional
characters of the types of hybridization discussed above are summarised in the
following Figure:
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.
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