CHEMICAL BONDING AND MOLECULAR STRUCTURE 2

Question 1

what are the limitations of the octet rule : 1 and 2

Answer 1

The incomplete octet of the central atom
In some compounds, the number of electrons
surrounding the central atom is less than
eight. This is especially the case with elements
having less than four valence electrons.
Examples are LiCl, BeH2 and BCl3

.Li, Be and B have 1,2 and 3 valence electrons
only. Some other such compounds are AlCl3
and BF3

Odd-electron molecules
In molecules with an odd number of electrons
like nitric oxide, NO and nitrogen dioxide,
NO2
, the octet rule is not satisfied for all the
atoms

Question 2

what are the limitations of the octet rule : 3

Answer 2

Elements in and beyond the third period of
the periodic table have, apart from 3s and 3p
orbitals, 3d orbitals also available for bonding.
In a number of compounds of these elements
there are more than eight valence electrons
around the central atom. This is termed as
the expanded octet. Obviously the octet rule
does not apply in such cases.
Some of the examples of such compounds
are: PF5
, SF6
, H2
SO4
and a number of

coordination compounds.

Question 3

what are the drawbacks of the octet rule

Answer 3

It is clear that octet rule is based upon
the chemical inertness of noble gases.
However, some noble gases (for example
xenon and krypton) also combine with
oxygen and fluorine to form a number of
compounds like XeF2
, KrF2
, XeOF2
etc.,
* This theory does not account for the shape
of molecules.
* It does not explain the relative stability of
the molecules being totally silent about
the energy of a molecule.

Question 4

what does formation of ionic bond depend upon

Answer 4

• The ease of formation of the positive and
  negative ions from the respective neutral
  atoms;
• The arrangement of the positive and
  negative ions in the solid, that is, the
  lattice of the crystalline compound.

Question 5

when will the ionic bond be more easy to form

Answer 5

The formation of a positive ion involves
ionization, i.e., removal of electron(s) from
the neutral atom and that of the negative
ion involves the addition of electron(s) to the
neutral atom.

The electron gain process may be exothermic
or endothermic. The ionization, on the other
hand, is always endothermic. Electron affinity, is the negative of the energy change
accompanying electron gain

Obviously ionic bonds will be formed
more easily between elements with
comparatively low ionization enthalpies
and elements with comparatively high
negative value of electron gain enthalpy.

• low ionisation ehtlapy of metal
  -highly negative electron gain enthalpy of non metal
  -high lattice enthalpy

Question 6

how is arrangement of ions in ionic cpds?

Answer 6

Ionic compounds in the crystalline
state consist of orderly three-dimensional
arrangements of cations and anions held
together by coulombic interaction energies.
These compounds crystallise in different
crystal structures determined by the size of
the ions, their packing arrangements and
other factors.

Question 7

Lattice enthalpy plays a keyrole in understanding the ionic bonds epxlin

Answer 7

In ionic solids, the sum of the electron gain
enthalpy and the ionization enthalpy may be
positive but still the crystal structure gets
stabilized due to the energy released in the
formation of the crystal lattice. For example:
the ionization enthalpy for Na+(g) formation
from Na(g) is 495.8 kJ mol–1 ; while the electron
gain enthalpy for the change Cl(g) + e–
→
Cl– (g) is, – 348.7 kJ mol–1 only. The sum of the
two, 147.1 kJ mol-1 is more than compensated
for by the enthalpy of lattice formation of
NaCl(s) (–788 kJ mol–1). Therefore, the energy
released in the processes is more than the
energy absorbed. Thus a qualitative measure
of the stability of an ionic compound
is provided by its enthalpy of lattice
formation and not simply by achieving
octet of electrons around the ionic species
in gaseous state

Question 8

define lattice enthalpy

Answer 8

The Lattice Enthalpy of an ionic solid
is defined as the energy required to
completely separate one mole of a solid
ionic compound into gaseous constituent
ions.

the lattice enthalpy of
NaCl is 788 kJ mol–1. This means that 788
kJ of energy is required to separate one mole
of solid NaCl into one mole of Na+ (g) and one
mole of Cl– (g) to an infinite distance.

Question 9

how can lattice enthlpy be calculated

Answer 9

This process involves both the attractive
forces between ions of opposite charges
and the repulsive forces between ions of
like charge. The solid crystal being threedimensional; it is not possible to calculate
lattice enthalpy directly from the interaction
of forces of attraction and repulsion only.
Factors associated with the crystal geometry
have to be included.