A
ATOMIC HYBRIDAZATION
T
O
M
Hybridisation
may be defined as the phenomenon of intermixing of the orbitals of
I
slightly
different energies so as to redistribute their energies and to give new set
of orbitals of equivalent energy and shape. The new orbitals formed as a result
of hybridization are called hybrid or hybridized orbitals. Thus, to form four
equivalent bonds, one 2s and three 2p-orbitals of carbon hybridize and form four
new orbitals.such orbitals are called sp3 hybrid orbitals
The important characteristics of hybridisation are listed below:
(i) The number of hybridized orbitals formed is equal to the number of orbitals that
get hybridized.
(ii) The hybridized orbitals are always equivalent in energy and shape.
(iii) The hybrid orbitals are more effective in forming stable bonds than the pure atomic
orbitals.
(iv) The hybrid orbitals are directed in space in some preferred directions to have stable
arrangements.Therefore, the type of hybridization gives the geometry of the
molecule.
Depending upon the different combinations of s- and three p-orbitals, three types of
hybridizations are known.
Hybridization and concept of sigma and pi bond
HybridizationThe structures of different molecules can be explained on the basis
of hybridization. For e.g., in case of carbon, the ground state electronic
To explain the tetravalency of carbon, it was proposed that one of the electrons
from 2s filled orbital is promoted to the 2p empty orbital (2pz), which is in a higher
energy state. Thus, four half-filled orbitals form in the valence shell this accounts
for the bonding capacity of four carbon atoms.
This state is known as excited state and the configuration of carbon in the excited
state is:
The above configuration reveals that all the four bonds formed by carbon will not be
identical. For e.g., in the formation of CH4 molecule, one C-H bond will be formed
by the overlapping of 2s-orbital of C and 1s-orbital of H whereas the other three
C-H bonds will be formed by the overlapping of 2p-orbitals of C and 1s-orbital of H.
Therefore, all the bonds will not be equivalent.
But actually, in most of the carbon compounds, such as methane (CH4),
carbon tetrachloride (CCl4) etc., all the four bonds of carbon atom are equivalent.
The equivalent character of the bonds can be explained with the help of hybridisation.
sp hybridization
This involves the mixing of one s- and one p-orbital forming two sp-hybrid
orbitals.
The two sp-hybrid orbitals are oriented in a linear arrangement and bond angle
is 180°. For e.g., BeF2 involves sp-hybridization and is, therefore,
linear.2-hybridization">
sp2 hybridization
In this case, one s- and two p-orbitals hybridize to form three sp2 hybrid orbitals.
These three sp2 hybrid orbitals are oriented in a trigonal planar arrangement.
For e.g., in BH3 boron atom undergoes sp2hybridization and therefore,
BH3 has trigonal planar geometry and HBH bond angle is 120o.3-hybridization"
sp3 hybridization
In this case, one s- and three p-orbitals hybridize to form four sp3 hybrid orbitals.
These four sp3-hybrid orbitals are oriented in a tetrahedral arrangement.
The common example of molecule involving sp3-hybridisation is methane (CH4).
Therefore, CH4 has tetrahedral geometry and HCH bond angle is 109.5o
Valence Shell Electron Pair Repulsion
VSEPR
Valence shell electron pair repulsion, VSEPR, is a super-simple technique for
predicting the geometry of atomic centres in small molecules and molecular ions:
Crucially, atomic centres with VSEPR determined geometry can be joined together
into molecular entities like cyclohexane and glucose:
The VSEPR Technique
Six or so steps are required to generate the VSEPR geometry of an atomic centre
such as:
Carbon in methane, CH4
Nitrogen in ammonia, NH3
Xenon in xenon tetrafluoride, XeF4
Iodine in the iodide difluoride ion, [IF2]–
First, determine the number of electrons in the outer (valence) shell about the
central atom (C, N, Xe, I, etc.)
Second, find valency and number of electrons associated with the ligand X
Third, construct a valid Lewis structure of the molecule in question showing
all of the bonds and all of the lone pairs (nonbonded pairs) of electrons.
If the structure is a molecular ion, add one valence electron for each negative
charge and remove one valence electron for each positive charge.
Not all Lewis structures have eight electrons about the central atom A
(as emphasized by very simple Lewis octet theory). For example,
Sulfuric acid, H2SO4, has two monovalent OH functions and two doubly bonded
oxygens that behave as single ligands:
Phosphorus pentachloride, PCl5, has 10 electrons:
Fourth, determine the "total coordination number" of the central atom, where:
total coordination number =
number of electron pairs =
number of electrons in outer shell divided by 2
Methane, CH4, ammonia, NH3, the ammonium ion, [NH4]+ and the nitranion
(amide ion), [NH2]–, [above] all have eight electrons in the valence shell of the
central atom and all have a total coordination number of 4.
Fifth, the overall geometry of the atomic centre is determined by the mutual
repulsion between the electron pairs of the total coordination number.
The effect can be replicated by holding 2, 3, 4, 5 or 6 balloons together:
2 Balloons give a linear geometry
3 Balloons give a trigonal planar geometry
4 Balloons give a tetrahedral geometry
5 Balloons give a trigonal bipyramidal geometry
6 Balloons give an octahedral geometry
Sixth, there two adjustments are required by the VSEPR method to find the geometry
of an atomic centre:
Lone pairs of electrons (nonbonded pairs) are taken into account in determining the
total coordination number and VSEPR geometry, but they are NOT used when
defining the geometry of an atomic centre, only the atoms are used:
Lone-pairs of electrons (blue) behave as if they are slightly bigger than bonded
electron pairs (green) and act to distort the geometry about the atomic centre so
that bond angles are slightly smaller than expected:
Methane, CH4, has a perfect tetrahedral bond angle of 109° 28' (109.47°),
while the H-N-H bond angle of ammonia, H3N:, is slightly less at 107°
The oxygen of water has two bonded electron pairs (green) and two nonbonded
"lone" electron pairs (blue) giving a total VSEPR coordination number of 4.
But the geometry is defined by the relationship between the H-O-H atoms and
water is said to be "bent" or "angular" shape of 104.5°.
Some Words About The Nature of VSEPR "Theory"
Valence shell electron pair repulsion – extraordinary though this may
sound – is not based on any deep theory. It is simply a 'neat trick' that works
extraordinarily well.
VSEPR 'theory' only says that ligands arrange themselves about an atomic
centres so as to maximise spherical symmetry.
The 'theory' does not in any way explain why non-bonding lone pairs of
electrons behave as they do.
It is often said that the VSEPR structure of water shows "two lone pairs of
electrons sticking out like a pair of rabbit's ears"