Limitations of Octet Rule
(a) The rule failed to predict the shape and relative stability of molecules.
(b) It is based upon the inert nature of noble gases. However, some noble gases like xenon and
krypton form compounds such as XeF2 , KrF2 etc
.
(c) The octet rule cannot be applied to the elements in and beyond the third period of the periodic
table. The elements present in these periods have more than eight valence electrons around the
central atom. For example: PF5 , SF6 , etc.
(d) The octet rule is not satisfied for all atoms in a molecule having an odd number of electrons. For
example, NO and NO2 do not satisfy the octet rule.
(e) This rule cannot be applied to those compounds in which the number of electrons surrounding
the central atom is less than eight. For example, LiCl, BeH2, AlCl3 etc. do not obey the octet rule.
BOND LENGTH
Bond length is the average distance between the centres of the nuclei of two bonded
atoms in a molecule. It is expressed in Angstrom units (Å) or picometers (pm).
1Å = 10-10m and 1pm = 10-12 m. It is determined with the help of X-rays diffraction and
other spectroscopic methods.
Bond length depends upon the following factors:
Bond Multiplicity Bond length decreases with increase in bond multiplicity.
CC bond length is shorter than C=C bond which in turn is shorter than C-C.
Size of the Atom The bond length increases with increase in the size of the atom. From the
above values it is clear that the bond lengths for a given family increase with increase in
atomic number. For example : C-C < Si-Si < Ge-Ge
This is because with the increase in size of the atom, the distance of the electrons from the
nucleus increases successively with the addition of a new shell. Therefore the average
distance between the bonding nuclei (bond length) increases.
BOND ANGLE
It is defined as the average angle between the orbitals of the central atom containing the
bonding electron pairs in the molecule. It is expressed in degree/minute/second. This gives
an idea about the distribution of orbitals around the central atom in a molecule. Therefore
bond angle determines the shape of a molecule.
For example, the H-O-H bond angle in H2O is 104.5° and H-N-H bond angle is NH3 107°.
Concept 2. BOND PARAMETERS
BOND DISSOCIATION ENTHALPY
It is defined as the enthalpy change involved to break one mole of bonds of a particular
type between the atoms of a molecule in the gaseous state. It is expressed in terms of kJ
mol-1. When a bond is formed between the atoms, energy is released and the bonded
atoms have lesser energy than the separated individual atoms. Then, same amount of
energy will be needed to form the bond. This energy is called the bond dissociation energy
and is a measure of bond strength. Larger the bond dissociation energy, stronger will be
the bond in the molecule.
Bond Dissociation Enthalpy depends upon:
Size of Bonded Atoms
Bond Length
Bond Polarity
BOND ORDER
Bond order is a measurement of the number of electrons involved in bonds between two
atoms in a molecule.
Most of the time, bond order is equal to the number of bonds between two atoms.
Exceptions occur when the molecule contains antibonding orbitals.
Bond order is calculated by the equation:
Bond order = (number of bonding electrons - number of antibonding electrons) / 2
If bond order = 0, the two atoms are not bonded.
Examples:
The bond order between the two carbons in acetylene is equal to 3.
The bond order between the carbon and hydrogen atoms is equal to 1.
There is no direct relationship between the formula of a compound and the shape of its
molecules. The shapes of these molecules can be predicted from their Lewis structures,
however, with a model developed about 30 years ago, known as the valence-shell
electron-pair repulsion (VSEPR) theory.
The VSEPR theory assumes that each atom in a molecule will achieve a geometry that
minimizes the repulsion between electrons in the valence shell of that atom. The five
compounds shown in the figure below can be used to demonstrate how the VSEPR theory
can be applied to simple molecule.
This theory was proposed for the first time by Sidgwick and Powell in 1940 and developed
by Gillespie and Nyholm in 1957. According to this theory
“The shape of a given species (molecule or ion) depends on the number and nature of
electron pairs surrounding the central atom/ion of the species
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