CHEMICAL BONDING AND MOLECULAR STRUCTURE 3 Flashcards
define bond length
Bond length is defined as the equilibrium
distance between the nuclei of two bonded
atoms in a molecule. Bond lengths are
measured by spectroscopic, X-ray diffraction
and electron-diffraction techniques
what is covalent radius
The covalent radius is measured
approximately as the radius of an atom’s
core which is in contact with the core of an
adjacent atom in a bonded situation. The
covalent radius is half of the distance between
two similar atoms joined by a covalent bond in the same molecule.
what is van der waal’s radius
The van der Waals
radius represents the overall size of the
atom which includes its valence shell in a
nonbonded situation. Further, the van der
Waals radius is half of the distance between
two similar atoms in separate molecules in
a solid.
what is bond angle
It is defined as the angle between the orbitals
containing bonding electron pairs around the
central atom in a molecule/complex ion. Bond
angle is expressed in degree which can be
experimentally determined by spectroscopic
methods. It gives some idea regarding the
distribution of orbitals around the central
atom in a molecule/complex ion and hence it
helps us in determining its shape.
what is bond enthalpy
It is defined as the amount of energy required
to break one mole of bonds of a particular
type between two atoms in a gaseous state.
The unit of bond enthalpy is kJ mol–1. For
example, the H – H bond enthalpy in hydrogen
molecule is 435.8 kJ mol–1
. For example in case of H2O
molecule, the enthalpy needed to break the
two O – H bonds is not the same.. explain
H2O(g) → H(g) + OH(g); ∆aH1
= 502 kJ mol–1
OH(g) → H(g) + O(g); ∆aH2
= 427 kJ mol–1
The difference in the ∆aH value shows
that the second O – H bond undergoes
some change because of changed chemical
environment. This is the reason for some
difference in energy of the same O – H bond
in different molecules like C2H5OH (ethanol)
and water.
what is bond order?
In the Lewis description of covalent bond,
the Bond Order is given by the number
of bonds between the two atoms in a
molecule
what is he bnon order of isoelectronic molecules
Isoelectronic molecules and ions have
identical bond orders; for example, F2 and
O2
2– have bond order 1. N2, CO and NO+ have
bond order 3.
A genera l corre lat ion use fu l for
understanding the stablities of molecules
is that: with increase in bond order,
bond enthalpy increases and bond length
decreases.
what is resonance/
. According to the concept
of resonance, whenever a single Lewis
structure cannot describe a molecule
accurately, a number of structures with
similar energy, positions of nuclei, bonding
and non-bonding pairs of electrons are
taken as the canonical structures of the
hybrid which describes the molecule
accurately. This hybrid which represents the molecule structure accurately is called hybrid molecule.Resonance is
represented by a double headed arrow.
why do some cpds show resonance
Resonance stabilizes the molecule as the
energy of the resonance hybrid is less
than the energy of any single cannonical
structure; and,
* Resonance averages the bond
characteristics as a whole.
Explain the structure of CO3
2– ion in
terms of resonance.
The single Lewis structure based on
the presence of two single bonds and
one double bond between carbon
and oxygen atoms is inadequate to
represent the molecule accurately as it
represents unequal bonds. According
to the experimental findings, all carbon
to oxygen bonds in CO3
2– are equivalent.
Therefore the carbonate ion is best
described as a resonance hybrid of the
canonical forms I, II, and III
Explain the structure of CO2 molecule
The experimentally determined carbon
to oxygen bond length in CO2 is
115 pm. The lengths of a normal
carbon to oxygen double bond (C=O)
and carbon to oxygen triple bond (C≡O)
are 121 pm and 110 pm respectively.
The carbon-oxygen bond lengths in
CO2 (115 pm) lie between the values
for C=O and C≡O. Obviously, a single
Lewis structure cannot depict this
position and it becomes necessary to
write more than one Lewis structures
and to consider that the structure of
CO2
give the bond angles for the following molecules
i) water
ii) ammonia
iii)methane
iv)carbn dioxide
v) boron triflouride
i) 104.5
ii) 107
iii) 109.5
iv) 180
v) 120
what are fajan’s rules
- The smaller the size of the cation and the
larger the size of the anion, the greater
the covalent character of an ionic bond. - The greater the charge on the cation, the
greater the covalent character of the ionic
bond. - For cations of the same size and charge,
the one, with electronic configuration
(n-1)dⁿs⁰
, typical of transition metals, is
more polarising than the one with a noble
gas configuration, ns2 np6, typical of alkali
and alkaline earth metal cations.
how does ionic bond become covalent
The cation polarises the anion, pulling
the electronic charge toward itself and
thereby increasing the electronic charge
between the two. This is precisely what
happens in a covalent bond, i.e., buildup
of electron charge density between the
nuclei.
