CHEMICAL BONDING AND SHAPE
OF MOLECULES
CLASS 11
Few introductory terms
Valence shell
Outermost shell of an atom of an element is called valence shell
Valence electrons
The electrons present in the outermost shell of an atom is called
valence electrons.
In the most of the cases only
valence electrons take part in
chemical combinations.
Li, Na, K have one valence electron
Core electrons
Except valence electrons , the rest of the electrons in an atom are
called core electrons
In general core electrons do not take part in the formation of
chemical bond
Valency
According to classical concept valency is defined as combining
capacity of an element.
For eg. Valency of nitrogen in NH3 is 3 and
valency of copper in CuO is 2
According to electronic concept valency of an element is the
number of electron donated or accepted or shared by an atom
during formation of chemical bond.
For eg. In NaCl
valency of Na and Cl is 1
Octet Rule
He, Ne , Ar, Kr, Xe are called noble or inert gases. All noble
except helium have eight electrons in their valence shell. They
are chemically inert (stable ).
Most of the atoms tend to acquire eight electrons ( octet state)
in their valence shells to become stable and this is the cause of
chemical combination. This rule is known as Octet rule.
Lewis electron symbol or Lewis dot symbol
G.N. Lewis introduced simple notations to represent valence
electrons in an atom. These notations are called Lewis electron
symbol or electron dot or cross symbol
The letters represent the core electrons and nucleus whereas
dots around letters represent the valence electrons.
H
or
H
Na
Or
C
or
Na
Chemical bond
It is defined as the attractive force which binds various atoms or
ions together in a species.
Whatever the molecule is formed chemical bond is said to be
formed.
The formation of bond between two atoms primarily involves
rearranging their valence electrons by lowering energy to form
more stable arrangement.
Types of bonds
Depending upon their mode of formation there are three
types of primary bonds. These are
1. Electrovalent bond or ionic bond
2. Covalent bond
3. Co- ordinate covalent bond or Co-ordinate bond
Some secondary bonds are
1. Hydrogen bond
2. Vander Waal's force of attraction
Electrovalent bond or ionic bond
The bond formed by complete transfer of one or more electrons
from one atom to the another atom is known as ionic bond.
In another words Ionic bond is the electrostatic force of
attraction between the two oppositely charged ions.
The number of electrons lost or gained by atoms during
formation of ionic bond is called electrovalency of that atom
These type of bond is usually formed between metal and non
metal.
The compounds containing ionic bonds are called ionic
compounds
For example. NaCl, KCl, MgCl2, CaO, CaF2, Na2S , Al2O3
Formation Of NaCl
Lewis dot structure
Formation of MgCl2
Lewis dot structure
Q. Draw the Lewis structure of following ionic
compounds
AlF3
KF
MgS
CaO
Na3P
CaCl2
Al2O3
Favourable conditions for the formation of ionic bond
1.
Lesser the ionization energy of atom( usually metal), greater
is the ease of formation of cation and hence greater will be
the ease of formation of ionic bond. The alkali metals and
alkaline earth metal have low ionization energy so they form
ionic bond
2.
The non metal which accepts the electrons to form anion
should have high electron affinity. Greater the electron
affinity, higher will be the tendency of atom to form anion.
Halogens have highest electron affinity so they form ionic
compounds more easily.
3.
When cation and anion combine to form ionic compounds
,energy is released .It is called lattice energy. It may be
defined as the amount of energy released when one mole of
ionic compound is formed from its gaseous cations and
anions. Greater the lattice energy , greater will be the the
ease of formation of ionic bond.
4.
There should be large difference in electronegativities of
combining atoms (>1.8).
Characteristics of ionic compounds
1. Physical State
In ionic compound ions are arranged in well defined
patterns to form ionic crystal lattice. Ionic compounds are
hard and rigid because the cations and anions are held
together by strong electrostatic force of attraction
2. High melting and high boiling points.
Ionic compounds have high melting and boiling point because
the cations and anions are held by strong electrostatic force and
large amount of energy is needed to overcome these forces.
3.Electrical Conductivity
When ionic compounds are melted or dissolved in water, they
split into cations and anions and these ions are set free to move.
These ions carry electrical charges. Therefore, they conduct
electricity .But they cannot conduct electricity in solid state
because the ions occupy fixed positions and are not able to
move freely
4.Solubility
They are soluble in polar solvents like water and insoluble in non polar
solvents such as benzene (C6H6), Carbon tetrachloride (CCl4). When
ionic compounds are dissolved in water , the high dielectric constant of
water decreases the force of attraction holding the oppositely charged
ions. As a result , charged particles get separated and compound
dissolves.
5. Highly Brittle
Ionic crystals are made of layers of cation and anions in
alternative positions. When a little stress is applied to the
crystal, the ions with similar charge come together and begin to
repel each other . This tendency makes solid break easily . Hence
ionic solids are highly brittle.
6. Non-directional nature
Ionic bond is non-directional in nature. A cation and anions are
considered as charged spheres. These ions have uniform field of
force around them. These ions, thus ,attract each other. These
ions will thus attract oppositely charged ions from all directions.
Hence ionic bond is non directional and they do not show
isomerism
7.Fast Reactions
Ionic compounds undergo ionic reactions and these are very fast,
It is because no force is required to break the bonds of reacting
molecules.
Covalent Bond
The bond formed by mutual sharing of electrons between the
combining atoms.
The number of electrons contributed by an atom for sharing
during the covalent bond formation is called covalency.
The compounds containing covalent bonds are called covalent
compounds.
For example: H2, Cl2, O2, N2,HCl, H2O, NH3, CO2, CH4, C2H6, C2H4
Lewis dot structure of some covalent molecules
H2 Molecule
Cl2 Molecule
O2 Molecule
Formation of N2
Formation of C2H6
Formation of C2H4
Formation of CO2
Favourable conditions for the formation of covalent
bond
High ionization energy
Both participating elements should have high ionization energy.
The atoms which have high value of ionization energy are
incapable of losing electrons to form cation. Thus, these
elements can not form ionic bond but can form covalent bond
High electron affinities
Generally both combining atoms should have high electron
affinities, such that they have low capacity of losing electrons.
None of these elements will tend to lose electrons. In such a
case , they will fulfill their nearest noble gas configuration by
sharing electrons
Electronegativity difference
Both atoms should have low electronegativity difference( <1.8 in
Pauling scale ). so that the transfer of electrons from one atom
to another may not take place. Therefore , generally non metals
(C,H,O,N, S, P, F, Cl, Br, I) combine with non metals giving a
covalent bond
Number of valence electrons
Each of two atoms should have 4, 5, 6, 7 valence electrons (
except H) so that both the atoms acquire stable octet by sharing
3,2,1 electron pair.
High nuclear charge and small atomic size
High nuclear charge and small atomic size attract the valence
electrons more tightly and prevent loss of electrons. In such
condition sharing of electron take place.
Properties of covalent compounds
1. Physical state
Covalent compounds may exist in solid, liquid and gaseous state
For example
✔ H2O, H2O2 , CH3OH – liquid
✔ H2, N2 ,NH3, O2 - Gas
✔ Diamond and Graphite- solid
2. Low boiling and low melting points
The covalent compounds consist of individual molecules which
are held by weak Vander Waals forces. These weak forces are
easily overcome on application of heat. Hence , these
compounds melt and boil at low temperature
3. Electrical Conductivity
Covalent compounds are bad conductors of electricity due to
absence of free ions in the fused state or aqueous solution
4. Solubility
Covalent compounds are generally insoluble in polar solvent like
water but dissolve (soluble)in non polar organic solvent like
benzene, ether etc . However, some of them like alcohol,
ammonia are soluble in water due to formation of H- bond.
5. Low rate of reaction
Covalent compounds undergo molecular reactions which are
slow.it is because , high energy is required to break the covalent
bond and reaction involve breaking of old bond and formation of
new bond.
6. Directional Character
Covalent bond is rigid and directional because the shared pair of
electrons is present in fixed position between combining atoms.
They can give rise to different arrangement of atoms in space. So,
the same molecular formula of covalent compound may represent
a number of different compounds with different properties. It
means covalent compounds able to show isomerism.
Co- ordinate covalent bond or
Co- ordinate bond or Dative bond
The bond formed by sharing of electrons between combining
atoms, in which both the shared electrons are contributed by
one of the bonded atoms only , is called Co-ordinate bond.
It is special case of covalent bond also known as dative bond or
semi polar bond.
• In this type of bonding, the atom that shares an electron
pair from itself is termed as the donor.
• The other atom which accepts these shared pair of
electrons is known as a receptor or acceptor.
• The bond is represented with an arrow →, pointing
towards acceptor from the donor atom.
• After sharing of electron pair each atom gets stability.
Some Examples of coordinate compounds
Formation of H3O+
Formation of NH4+
Formation of SO2
Formation of SO3
Formation of NH3..BF3
Properties of Coordinate Compounds
Physical state:
They are generally liquids(HNO3) and gases( SO2), some are
solids (NH4Cl)
Melting and boiling points:
This type of bond is weaker than Ionic bonding but slightly
stronger than covalent bond. The melting and boiling points of
such compounds is comparatively higher than that of covalent
compounds and lower than that of electrovalent or ionic
compounds.
Electrical conductivity:
Since they have semi-polar nature, they are poor conductors of
electricity.
Solubility:
They are sparingly soluble in water due to semi-polar behavior
but soluble in organic solvents(non polar).
Directional character:
Sharing of electrons takes place in a definite direction, hence, it
is a directional bond. The co-ordinate covalent bond is also rigid
and directional and therefore, the structure of such compounds
holds out possibilities for space. Hence some of these
compounds exhibit isomerism.
Reaction rate
Slow reaction
Lewis dot structure of some important species
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
11)
12)
13)
HOCl
HClO3
CO
H2SO3
H2SO5
H2 S 2 O 7
H3PO4
N 2 O4
N 2 O5
N2O
NO
N 2 O3
NO2
14)
15)
16)
17)
18)
19)
20)
21)
22)
23)
24)
25)
26)
H2SO4
HNO3
H2CO3
HClO4
COCl2
03
CO3- SO4- NO3 PO43HPO42S2O32HCO3-
27)
28)
29)
30)
31)
32)
33)
34)
35)
36)
37)
38)
39)
40)
H2 O 2
CCl4
P 2 O5
KClO3
KNO3
(NH4)2SO4
Na2SO4
Na2S2O3
CaCO3
Na2CO3
NH4NO3
NH4Cl
CaCl2
MgCl2
Resonance
Some molecules have two or more than two lewis structure of
equivalent energy. These structures are known as resonance
structure or canonical structure of the molecule.
This phenomenon is known as resonance
An intermediate structure among the resonance structure is
called resonance hybrid.
Resonance hybrid is most stable structure which is real structure
of molecule
For example O3, SO2, SO3, CO3- - , SO4- - , NO3- , PO4resonance
- -
show
Ozone (O3)
Sulphur dioxide (SO2)
Sulphur trioxide (SO3)
NO3 -
CO3- -
SO4- -
PO4 - -
Exception to octet rule
Many stable covalent molecules have central atoms that do not
have eight electrons in their Lewis structures. Some are
1. Electron-deficient Molecules
Some stable molecules contain central atom which have less
than 8 electrons in the valence shell.
2. Hypervalent Molecules
Elements in the third and higher periods (n ≥ 3) have more than
four valence orbitals and can share more than four pairs of
electrons with other atoms because they have empty d orbitals
in the same shell. Molecules formed from these elements are
sometimes called hypervalent molecules, such as PCl5, and SF6.
In PCl5, the central atom, phosphorus, shares five pairs of
electrons. In SF6, sulfur shares six pairs of electrons.
3. Odd-electron Molecules
There are a number of molecules whose total number of valence
electrons is an odd number. Molecules that contain an odd
number of electrons are called odd electron molecules.
Examples of stable odd-electron molecules are NO, NO2, and
ClO2.
Hydrogen Bond
The electrostatic force of attraction between covalently bonded
hydrogen atom with highly electronegative atom( such as F, O, N)
and highly electronegative atom of same or different molecule
is known as hydrogen bond
Conditions for hydrogen bonding
The molecule must contain a highly electronegative
atom
linked to H-atom . The higher the electronegativity, more is the
polarization of H atom and formation of hydrogen bonding.
The size of electronegative atom should be small . The smaller
the size , the greater is the electrostatic attractions. Chlorine
having the same electronegativity as that of nitrogen but
chlorine does not form hydrogen bond due to large size.
Thus only F, O, and N atoms can form hydrogen bonds, as
these atoms are small in size and have high electronegativities .
Types of hydrogen bonding
1. Intermolecular hydrogen bonding
The hydrogen bond formed between H atom of one molecule
and highly electronegative atom such as F, O, N of another
molecule of same or different substance is known as
intermolecular H- bond.
Hydrogen bond in HF
Hydrogen bond in water
Hydrogen bond in alcohol
Hydrogen bond in acetic acid
Hydrogen bond in ammonia
2. Intramolecular hydrogen bond
Hydrogen bond formed between the hydrogen atom and highly
electronegative atom like F, O, N present in the same molecule is
called intramolecular hydrogen bond .
Effect of hydrogen bond
1. Melting Point and boiling points
The compounds having hydrogen bonding show abnormally high
melting and boiling points. The high melting and boiling point of
the compound containing hydrogen bonds is due to the fact that
some extra energy is needed to break these bonds.
Thus hydrides of Fluoride (HF), Oxygen (H2O) and nitrogen (NH3)
have abnormally high melting and boiling points compared to
other hydrides of same group which form no hydrogen bonds
2. Association of molecules
Formation of aggregates containing two or more molecules due
to weak electrostatic interaction such as hydrogen bonding is
called molecular association of molecule. For example , water
molecules undergo molecular association due to H –bonding.
The molecules of carboxylic acids exist as dimer because of the
hydrogen bonding. The molecular masses of such compounds
are found to be double than those calculated from their simple
formula.
3. High solubility of some covalent compounds in water
The covalent compounds which can form hydrogen bonds with
water dissolve in it. For example lower alcohols (CH3OH,
CH3CH2OH), ammonia, etc dissolve in water because they form
hydrogen bond with water.
4. Physical state
Hydrogen bonds affect physical state of the substance
For example,
NH3 is liquid but PH3 is gas
H20 is liquid but H2S
In case of water, hydrogen bonding causes association of the H2O
molecules . Strong hydrogen bonding in H2O brings H2O molecules
close enough to form liquid .On the other hand H2S could not
form hydrogen bonding . H2S molecules are attracted to each other
by weak Van der Waals force. Therefore , H2O is a liquid with high
boiling point and So H2S is gas with low boiling point.
5. Viscosity and Surface tension
The resistance to flow is called the viscosity. Compounds in
which molecules are associated by strong hydrogen bonds have
higher values of viscosity and surface tension.
Honey, conc. sulphuric acid, glycerine (glycerol) are highly
viscous liquids due to larger number of H-bonds.
6. Density of ice is less than water
In ice each oxygen atom is tetrahedrally surrounded by four
other hydrogen atoms. Out of the four hydrogen atom two are
covalently bonded and other two by H-bonding. Due to this
arrangement, ice attains cage like structure with many empty
space in it . These empty spaces increase the volume of ice and
hence density decreases. When ice is heated H- bonds get
broken and cage like structure collapse. This make water
molecules more closer together . Thus liquid water occupy less
volume and hence higher density than ice.
At 4 °C almost all hydrogen bonds break. Normal expansion of
water takes place above 4 °C . Hence pure water has maximum
density at 4 °C