Electronic Theory of Valency
The chemical behaviour of an atom is determined to a large extent by the number and the arrangement of electrons
in the
outer orbitals of the atom. Only these electrons are involved in chemical combination and so these are called
the
valence electron .
Completed Electron Octet Or Duplet
Group
zero of the periodic table contains the nobal gases. With the exception of
helium which has a 1s2
electron arrangement others have ns2 np6 configuration in the outer orbitals.
He 1s2
Ne 1s
2 2s2 2p6
Ar 1s
2 2s
2 2p
6 3s2 3p6
Kr 1s
2 2s
2 2p
6 3s
2 3p
6 3d
10 4s2 4p6
Xe 1s
2 2s
2 2p
6 3s
2 3p
6 3d
10 4s
2 4p
6 4d
10 5s2 5p6
Since the atoms of the noble gases were not known to form chemical bond, it was argued that the
presence of 8 electrons (an electron octet) in the valence shell makes the atom stable. Therefore
all other atoms must
undergo bonding by gaining or losing or sharing electrons so as to aquire the
electronic configuration of the nearest inert gas .
Ionic bond or
Electrovalent bond is a bond formed by the complete
transfer of electrons from
one atom to another, so as to complete their valence shell with eight electrons (i.e. octet)
or two electrons (i.e. duplet). Atoms of
metals generally lose electrons and those of
non-metals gain electrons.
A Covalent bond is formed by the
mutual sharing of electrons between two atoms, each atom contributing
one electron to the
shared pair . A covalent bond is usually represented by a short line
(i.e, a dash)
between the atoms.
Lewis Structure
A Lewis symbol is a symbol in which the electrons in the valence shell of an atom
or simple ion are represented by
dots placed around the letter symbol of the element.
Each dot represents
one electron .
Hydrogen 1s
1
Oxygen 1s
2 2s
2 2p
4
We have seen that the formation of a covalent bond between two atoms, each atom contributes
one electron to the shared pair. Sometimes
both the electrons of the shared pair may come from one of the atoms.
The covalent bond thus formed is called a
co-ordinate bond or
dative bond .
Polar Covalent Bonds
Covalent bonds in which the sharing of the electron pair is
unequal , with the electrons
spending more time around the more nonmetallic atom, are called
polar covalent bonds .
In such a bond there is a charge separation with one atom being slightly more positive
and the other more negative, i.e., the bond will produce a
dipole moment. The ability
of an atom to attract electrons in the presense of another atom is a measurable property
called
electronegativity .
The phenomenon where a molecule can be represented by more than one structural formula
is known as
resonance .
VSEPR theory is used to predict the shapes of molecules. This theory predicts that bonding
(sigma bonds only) and non-bonding electron pairs in a molecule will adopt a geometry in
which the distance between the electron pairs is maximized from one another in order to
minimize the repulsions. This will result in a molecular geometry with the lowest possible energy.
The theory also allows us to predict which hybridization the central atom takes in bonding to other atoms.
For this, we count how many pairs of electrons (triple bonds are counted as one pair) are around the central atom.
If there are
two pairs of electrons, they must be positioned 180° apart from each other and the
shape is therefore
linear.
Three pairs are best positioned 120° apart and the shape is thus
trigonal planar.
Four pairs of electrons
are best positioned as
tetrahedral shape. For
five pairs of electrons, the shape is predicted to be
trigonal bipyramidal.
Last, the
octahedral is the shape predicted for
six pairs of electrons.
|
Molecule
|
Electron pairs
|
Shapes with, and without non-bonding e- pair
|
Hybridization of central atom
|
|
BeH2
|
2
|
linear, linear
|
sp
|
|
BF3
|
3
|
trigonal planar, trigonal planar
|
sp2
|
|
CH4
|
4
|
tetrahedral, tetrahedral
|
sp3
|
|
NH3
|
4
|
tetrahedral, trigonal pyramidal
|
sp3
|
|
H2S
|
4
|
tetrahedral, bent
|
sp3
|
|
PF5
|
5
|
trigonal bipyramidal, trigonal bipyramidal
|
dsp3
|
|
BrF3
|
5
|
trigonal bipyramidal, T-shaped
|
dsp3
|
|
TeCl4
|
5
|
trigonal bipyramidal, Seesaw
|
dsp3
|
|
SF6
|
6
|
octahedral, octehedral
|
d2sp3
|
|
XeF4
|
6
|
octahedral, square planar
|
d2sp3
|
|
XeF2
|
5
|
trigonal bipyramidal, linear
|
dsp3
|
In the molecular orbital approach, all of the electrons are present in the molecular orbitals.
These molecular orbitals are formed by linear combination of atomic orbitals (LCAO) .
Thus it is also known as LCAO -MO method.
The following are the essential features of the M.O Theory -
(i) In the M.O. model, all the electron are taken together and considered as moving in the field
of all the nuclei.
(ii) The atomic orbitals are combined to form, which are called molecular orbitals and electrons
are fed into these orbitals.
(iii) The number of combining atomic orbitals is equal to the number of molecular orbitals formed.
(iv) When two atomic orbitals combine, two M.O's are formed, of which one has a lower energy, while
the other has a higher energy. The former one is known as the bonding orbital and the latter one is known as the antibonding orbital .
A hydrogen atom normally forms a
single bond . In some compounds, however, the hydrogen atom may be
located between two atoms acting as a
bridge between them. Hydrogen atom is now involved in
two bonds,
one a normal
covalent bond , the other a
hydrogen bond. A hydrogen bond is always formed between two
small,
strongly
electronegative atoms such as fluorine, oxygen and nitrogen.
There are two types of hydrogen bond :-
(i) Intermolecular Hydrogen Bond .
(ii) Intramolecular Hydrogen Bond .