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Wednesday, 21 October 2015

FSc Notes Chemistry Part 1 Chapter 6 Chemical Bonding Lecture 6

FSc Notes Chemistry Part 1 Chapter 6 Chemical Bonding Lecture 6

Hybridization:

The process in which atomic orbitals of different shape and energy mix and form an equivalent no. of new atomic orbitals of same shape and energy is known as hybridization. These new orbitals are known as hybrid orbitals. The process of hybridization got its consideration when some problems were faced in case of bonding of some atoms.

For example, the electronic configuration of carbon is
6C = 1S2 , 2S2 , 2P2
K=2 L=4 ( 2 paired & 2 unpaired )
As C has only 2 unpaired is in its valence shell, so it should always form only two bonds according to valence bond theory. But in all organic compounds C has formed four covalent bonds. It is because of the transference of one of the 2s electron to the empty 2Pz. Thus due to greater no. of unpaired electron ( 4 unpaired electron ) C gets more energetic and is said to be in the excited state ( high energy state ).
Now C can form 4 covalent bonds. But as in all the 4 bonds, one S orbital is involved whose shape is different from P orbitals and also its energy is lower ( being closer to nucleus ) than P orbitals, thus all 4 bonds will not be same ( same length and energy ). Therefore to avoid this problem and to form all bonds of same energy and length, carbon undergoes the process of hybridization after excitation and before bond formation.
The shape of a hybrid orbital is two lobed, one lobe is larger while the other one is smaller.ie The energy of a hybrid orbital is less than the energy of S orbitals so we can say that the hybrid orbitals are closer to nuclear as compared to P orbitals, while away from the nucleus as compared to S orbital.

Types:
Although there are several types of hybridization. But the following three are the most simple and main types.
  1. SP3 hybridization
  2. SP2 hybridization 
  3. SP hybridization
SP3 Hybridization:
The hybridization in which one & S and three P orbitals intermix and form four hybrid orbitals of same shape and energy, is known as SP3 hybrid orbitals.
In each SP3 hybrid orbital, the S character is only 25% while P character is 75 % ( ie ¾ th ) , therefore ( ¼ th ) these 4 SP3 hybrid orbitals will not be very much close to the nucleus, as they will be near to “ P “ and away from S orbital.
i.e lets take the case of carbon.
These four SP3 hybrid orbital’s arrange themselves in a tetrahedral geometry. The angle between any two SP3 hybrid orbital’s is 109.5 degree.
Example: Methane ( CH4 ) : in CH4 molecule, carbon undergoes SP3 hybridization. Therefore CH4 has a tetrahedral geometry with bond angle of 109.5 degree.

SP2 Hybridization:
The hybridization in which one S and two P orbitals of different shape and energy mix to form 3 hybrid orbital’s, is known as SP2 hybrid orbital.
In case of each SP2hybrid orbital, S character is 33.33% while P character is 66.66%, therefore each SP2 hybrid orbital will be a bit closer to the nucleus as compared to SP3 hybrid orbitals, due to increase of S character hence the sigma bond formed from SP2 hybrid orbital will be stronger than the sigma bond formed by SP3nybrid orbitals and thus the SP2 hybridized. Carbon will more electronegative than the SP3hybridized carbon.
The three SP2 hybrid orbitals arrange themselves in a co—planner geometry. The angle b/w any two SP2 hybrid orbital’s is 120 degree.

Example:
Ethylene or Ethene Molecule ( C2H4 ) :
In ethane molecule each carbon undergoes SP2 hybridization. Thus the geometry of ethane molecule is co—planner and the bond angle is 120 degree. Two of the 2P orbitals ( ie 2px, 2py ) undergo hybridization and their shape and energy changes. But the 3rd orbital ie ( 2Pz ) remains unhybrid, so its shape and energy remain unchanged and it lies perpendicular to the plane of hybrid orbitals with one lobe above and one lobe below the hybrid orbital of one C overlaps with the SP2 hybrid orbital of other carbon, a sigma bond is formed b/w the two carbon atoms, which brings them so closer to each other that the unhybrid 2P2 orbitals of both the carbon atoms undergo a side-wise overlap resulting in the formation of a bond.

SP Hybridization:
The hybridization in which one S and one P orbital of different shape and energy mix and form two SP hybrid orbitals of same shape and energy , is known as SP hybridization . In each SP hybrid orbital, there is 50% P character. Thus due to the increased S character in case of SP hybridization , each SP hybrid orbital is more closer to the nucleus of the hybridized atom as compared to SP2 & SP3 , hybrid orbitals. The SP hybrid orbitals arrange themselves in a linear geometry and the angle between them is always 180 degree.

Example:
Acetylene or Ethyne ( C2H2 )
In acetylene molecule, each carbon undergoes SP hybridization. Thus the geometry of acetylene is linear and its bond angle is 180 degree .
As in SP hybridization, one S and only one P orbital get hybridized, thus the two P orbital’s i.e. 2Py & 2Pz orbitals of both the carbon atoms remain un-hybrid and they are perpendicular to the plane of hybrid orbitals as well as mutually perpendicular. The one SP hybrid orbital of one carbon atom overlap with the SP hybrid orbital of the other C atom forming a sigma covalent bond b/w them. Similarly the other SP hybrid orbital of each carbon overlap with the S orbital of hydrogen atoms producing two C—H sigma covalent bonds. Then due to the formation of sigma bond b/w c atoms, they come so close to each other that the 2Py of one C undergo a sidewise overlap with the 2Py of the other carbon & same case happens for 2Pz orbitals of both carbon atoms thus producing two Pi bonds b/w carbon atoms. Thus in acetylene molecule, there is a triple covalent bond b/w the carbon, atoms. Out of these three bonds , one is sigma covalent bond while the remaining two are Pi  covalent bonds. As the sigma bond is formed during linear overlap of the orbitals of carbon atoms , therefore it is much more stronger while the bonds are of equal strength and as they are formed by the side wise overlap of the orbital’s of c atoms therefore they are much weaker than the sigma bond. In case of ethane molecule ( C2H6 ) each carbon undergoes SP3 hybridization. Thus in each SP3 hybrid orbital, the S character is only 25%. In case ethane or ethylene molecule ( C2H4 ) each C undergoes SP2 hybridization. Here in each SP2 hybrid orbital, the S character is 33.3%. In case of ethyne molecule, ( C2H2 ) each C undergoes SP hybridization. In each SP hybrid orbital, the S character is 50%. As in case of ethyne ( C2H2 ) SP hybridization the S character is the maximum,thus each SP hybrid orbital is more closer to the nucleus of C atom and thus C atoms of ethyne are more electron negative than C atoms of ethane ( SP2 ) & ethane ( SP3 ). Therefore the C—H bond of ethyne is polar and thus ethyne & ethane are non—polar and thus ethyne is acidic in nature.

Paramagnetism:

Any substances which is attracted by the magnet, when placed in a magnetic field is known as paramagnetic substance and this phenomenon is known as paramagnetism. It is important to note that a paramagnetic substance must have one or more unpaired electrons in it. Greater the no. of unpaired electrons in a substance, greater will be its paramagnetic character and vice versa.
The fact that the unpaired electron is the responsible factor for the par magnetism of a substance, can be explained as:
The electron is a charged particle and it revolves ( spins ) a round its own axis and when a charged body spins around its own axis, it produces a magnetic field around itself. Thus we can say that an electron acts as a magnet.

Diamagnetism:

Any substance which is not attracted or repelled by a magnet when placed in a magnetic field is known as diamagnetic substance and the phenomenon is known as diamagnetism. It is important to note that a diamagnetic substance doesn’t have any unpaired electrons in it ie all its electron are paired.
When two electrons get paired in an orbital, their spin is always opposite according to Pauli Exclusion principle. Thus the two electrons ( paired ) in an orbital, produce equal and opposite magnetic fields which cancel out each other and thus the substance doesn’t have any magnetic character.


Written by: Asad Hussain

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