Chemical
Bond (Intramolecular Force) and Nature of Chemical Bond
Most atoms are not
stable and are not capable of independent existence. That is why substances
composed of single atoms are very rare and their examples include only few
monoatomic elements like He, Ar, Kr, Ne, Xe. The majority of the substances
around us is generally made up of compounds and mixtures (which themselves are
built of elements). Obviously, there must be some force which holds atoms
together in a molecule or crystals otherwise the atoms would simply fly apart
and no compound could exist.
“The linkage or force
which holds atoms together in a molecule or a crystal is called Chemical Bond
or Intramolecular Force.” OR “The attractive force which binds atoms together
in the form stable molecule or a crystal (comprising of formula units) of a
compound is called Chemical Bond.”
The chemical bonds
are electrical in nature involving
mutual attraction between unliked charged species. A stable molecule is only
formed when the total energy of the combined atoms in the molecule is less than
the total energy of the isolated (separate) atoms, so energy is evolved during bond formation. Lower the energy, stable
will be the molecule. Thus bond making is always exothermic.
Valence
Shell and Valence Electrons
The part of the atom involved in the formation of
chemical bond is the outermost shell or valence shell containing valence
electrons. The electrons in the outermost shell (i.e. the highest energy orbit)
of an atom are called the Valence Electrons. The valence electrons are farthest
from the nucleus and are thus considered to be the most loosely held electrons.
Only the valence electrons are involved in the formation of chemical bonds (and
the inner or core electrons do not take part in bond formation). Every element
exhibits a large increase in ionization energy when core-electrons are removed
from its noble gas core, supporting the idea that only the outermost electrons,
those beyond the noble gas core, are involved in the sharing and transfer of
electrons that give rise to chemical bonding and reactions. Thus the valence
shell electronic configuration of an atom controls the atoms’s chemistry.
All the elements in a particular group of the periodic
table possess the same number of valence electrons which are shown by their
group numbers.
No atom has more than 7 valence
electrons except noble gases Ne, Ar, Kr, Xe, Rn which have 8 valence
electrons (Noble gas He has only 2 valence electrons). e.g.
(1). alkali
metals of group IA (Li, Na, K, Rb, Cs) have 1 valence electron.
|
(2). alkaline
earth metals of group IIA (be, Mg,
Ca, Sr, Ba) have 2 valence electrons
|
(3). Elements of group IIIA (B,
|
(4). Elements of group IVA (C, Si, Ge, Sn,
Pb) have 5 valence electrons
|
(5). Elements of group VA (N, P, As, Sb,
Bi) have 5 valence electrons
|
(6). Chalcogens of group VIA (O, S, Se, Te)
have 6 valence electrons.
|
(7). Halogens of Group VIIA (F, Cl, Br, I)
have 7 valence electrons.
|
Need and
Reason for Bond Formation
1. Emission of
Energy To get lower energy for acquiring stability
The molecules become
stable when there is a net increase in the attractive force. This attractive
force results in the decrease in energy of the molecule. Lower the energy,
stable will be the molecule.
2. Octet Rule or
Duet Rule (acquiring inert gas or 8-electrons
configuration/
Noble gases formerly
called Inert Gases like Ne, Ar, Kr, Xe etc have 8 electrons in their outermost
(valence) shell. They are non-reactive and regarded as more stable. They have a
stable outer shell consisting of 8 electrons (octet). The lightest noble gas
helium has the stable outer shell of two electrons (duplet). According to Octet
Rule:
“The outermost shell
can have at maximum eight electrons and atoms tend to acquire an outermost
shell of eight. Some atoms (like hydrogen, lithium, and beryllium) tend to
attain an outermost shell of only two electrons like the lightest noble gas
helium. This is in accordance with the general rule that all processes tend to
move towards the state of maximum stability.”
It is a natural
tendency of all atoms to complete their valence shell so as to attain the
stable structure of their nearest noble gas i.e. an octet or a duplet of
electron in their outermost orbit. The atoms of elements attain stability when
they combine to form molecule in which each atom attains an inert gas like
stable electronic configuration in its valence shell. This tendency of
acquiring noble gas configuration is the basis of chemical bond formation.
Ways of Uniting Atoms
In the formation of
chemical bonds between atoms of elements, the valence electrons are either
transferred from the valence shell of one atom to the valence shell of another
atom or shared between them usually attaining a stable outer shell consisting
of 8 electrons (octet) except H2, Li, Be which have stable outer
shell of two electrons (duplet). In other words the stable noble gas
configuration can be achieved by following two ways:
(a) By Complete
Transference of One or More Electrons
The one way of
attaining stable noble gas configuration is the complete transfer of one or
more valence electrons from one atom to another. This usually happens when
atoms of metallic elements combine with atoms of non-metallic elements. Metals
being electropositive lose their valence electrons to form cations and
non-metals being electronegative gain those electrons to form anions. Both
acquiring stable outer shell of 8 (octet) are held together by strong
electrostatic force between them forming ionic bond.
(b) By Mutual
Sharing of Electrons
The second way of
attaining stable outer shell is the mutual sharing of one or more electrons
between atoms of different elements and atoms of similar elements to form
covalent bond.
The different methods
of acquiring an octet or duplet of electrons in the outermost shell to achieve
stable configuration of noble gas, gives rise to different types of chemical
bondings is the real basis of bond formation
Exceptions to Octet Rule (Failure or Limitations of Octet Rule)
Dr Inam Ul Haq Jazbi
Although, according
to octet rule, the central atom (as well as surrounding atoms) in most of the
covalent compounds attain 8 electrons, yet there are several covalent molecules
in which the central atom does not obey octet rule. The octet rule applies
mainly to the second-period elements of the periodic table which have only 2s
and 2p subshells that can hold a total of 8 electrons thereby attaining the
electronic configuration analogous to the noble gas neon. The second-period elements,
unlike third-period (or higher periods) elements, do not have 2d levels, so
they can never expand their valence shells and atoms of second-period elements
can never be surrounded by more than 8 electrons in any of their compounds.
Exceptions to the octet
rule fall into three categories:
1. Odd-Electron Molecules
2. An incomplete
Octet or Lessened Octets (Octet-deficient Molecules)
3. The expended
Octet or Valence shell expansion (Hypervalent Molecules)
?Dr Inam Ul Haq Jazbi
Types of Chemical Bonds
There
are three types of chemical bond i.e.
1.Ionic/Electrovalent
Bond
|
[by
the transfer of one or more electrons from one atom to another giving ions]
|
2.Covalent
bond
|
[by
the mutual sharing of one or more electrons between bonded atoms]
|
3.Co-ordinate
Covalent Bond
|
[by
the one sided sharing of an electron pair between bonded atoms].
|
4.Metallic
Bonds
|
[by
the electrostatic forces between metal cations and delocalized electrons]
|
Ionic Bond or Electrovalent Bond
?Dr Inam Ul Haq Jazbi
Definition of Ionic Bond
In
1916 a German scientist W. Kossel proposed the concept of ionic bond.
“The electrostatic
force of attraction arise due to complete transfer of one or more valence
electrons from one atom to another that binds oppositely charged ions (i.e.
positive and negative ions) together is termed as ionic or Electrovalent Bond.”According to Lewis theory, ionic
bond is formed by the complete transfer of electron (s) from an atom with low
I.P. to another with high E.A.
During the formation
of ionic bond, a metallic atom loses one or more valence electrons acquiring
positive charge and change into cation while a non-metallic atom gains these
electrons acquiring negative charge and changing into anions. The cation and
anion attaining the stable electronic configuration of nearest noble gas, being
oppositely charged are attracted towards each other and held together by
electrostatic forces acting between them. The cations and anions are mutually
surrounded by each other in an orderly arrangement and are associated to form
giant crystal lattice [releasing lattice energy which reduces the energy of
system and making it stable.]
Ionic bonds are
non-directional i.e. it extends equally in all directions. Due to
non-directional nature of ionic bonds, ionic crystals neither have any geometry
nor show stereo (space) isomerism.
Conditions for forming Ionic Bond
Ionic bonds are
usually formed between two dissimilar atoms of elements i.e. metals and
non-metals having an electronegativity difference of more than 1.7. Strong
electropositive elements of group IA and IIA and highly electronegative
elements of group VIA and VIIA combine to form ionic bonds e.g. NaF, NaCl,
NaBr, NaI, KF, KCl, KBr, KI, NaH, KH, LiF, MgO, CaO, Na2O, K2O,
KO2, Na2O2, NaOH, KOH, etc.
1.The atoms taking part in bond formation
should be dissimilar in nature i.e. oppositely charged particles.
2. The lowering of energy of system should take
place after bond formation.
Factors
affecting for Ionic Bond
1. High
Electronegativity Difference more than 1.7
2. Low
Ionization Potential and High Electron Affinity
3. Emission
of High Lattice Energy
1. High Electronegativity
Difference more than 1.7
Ionic bonds are
formed by the elements of low electronegativity (e.g. metals) and the elements
of high electronegativity (e.g. non-metals). A difference of electronegativity
value of about 1.7 between two elements leads to the formation of ionic bond.
2. Low Ionization
Potential and High Electron Affinity
The ionization
potential (I.P) of metal atom should be low so that electrons may easily be removed
from it to form cation. The metals of group I, II and III (Al) A due to their
low I.P. and low E.N., lose one, two or three electrons to form M+,
M2+ and M3+ ions respectively. While electron affinity
(E.A.) of non-metals should be high. The non-metals in group VIA and VIIA due
to their high E.A. and high E.N., tend to gain two and one electron to form A2–
and A1– ions respectively.
3. Emission of High
Lattice Energy
The energy released
when gaseous ions arrange to form crystal lattice is called Lattice Energy.
Higher the value of lattice energy of crystal, greater is the ease of formation
of ionic bond and greater will be stability of ionic compound.
?Dr Inam Ul Haq Jazbi
?Dr Inam Ul Haq Jazbi
Formation of Ionic Bond in Sodium Chloride
In order to form
ionic bond, ∆E.N. between bonded atoms must be greater than 1.7. The between Na
and Cl is 2.1 (3-0.9). Thus sodium metal and chlorine gas combine
electrovalently or ionically to form formula unit of an electrovalent compound,
sodium chloride (NaCl). The overall reaction can be shown as:
The formation of
ionic bond in sodium chloride involves the loss of one valence electron by
sodium atom to form monopositive sodium cation (Na+) with stable
octet and this electron lost by Na is picked up by a chlorine atom completing
its octet to form uninegative chloride anion (Cl–). Na+
and Cl– ions acquire neon and argon structure respectively. These
oppositely charged ions are held together by electrostatic forces of attraction
and are associated to form a crystal lattice.
The formation of
ionic bond in sodium chloride (NaCl) involves the following steps:
Energy changes
in formation of NaCl
The energy changes
are as follows:
This loss of energy
(147 kJ/mole) is more than compensated when oppositely charged ions i.e. Na+
and Cl– attract each other and are associated to form a crystal
lattice (consisting of a closely packed array in which each ion is surrounded
by six opposite ions), releasing lattice energy which can be defined as “the
energy released when one gram mole of gaseous ions arrange to form crystal
lattice”. The lattice energy compensates the loss of energy which occurs in the
formation of ions.
Release of high
lattice energy and great attractive forces in the crystal lattice greatly
reduce the energy of the system thereby making it stable. [The total decrease
in energy of the system is (+495 – 348 – 788 = 651 kJ/mole)].
Conclusion
Thus, it is essential
for the formation of ions and ionic bond that the sum of energies released in 2nd
and 3rd steps must be larger than what is required in the first
step.
Covalent
Bond or Electron Pair Bond
Definition
The idea of the electron pair bond or
covalent bond was first introduced by G.N. Lewis in 1916. When two or more
atoms of the same or different elements having same or nearly same
electronegativities react, transfer of electrons does not occur instead the
atoms attain noble gas structure by completing their valence shell by means of
sharing of unpaired valence electrons obeying octet rule and a covalent bond is
formed. In covalent bond each atom has to contribute equal no. of unpaired
valence electrons.
“The bond which is
formed by mutual sharing of one or more pairs of electrons called shared pair
of electrons that links the atoms in a molecule is known as Covalent Bond.”
OR
“A covalent bond is a
joint internuclear attractive force which is originated due to the mutual
sharing of unpaired valence electrons between the bonded atoms.”
Conditions for Forming Covalent Bond
The covalent bond is usually formed between
two same or different non-metallic elements (or between weak electropositive
and non-metallic elements) having same or nearly same electronegativities. ). [In such cases, atoms have no tendency to gain or lose electrons. Under these
conditions, transfer of electrons does not involve. Instead electrons are
shared equally by the two atoms, obeying Octet Rule]. The covalent bonding is
usually found in two non-metallic elements or atoms. According to Pauling’s
Electronegativity Scale, if the difference in the electronegativities of bonded
atoms is up to 1.7, then the bond is covalent bond. If the difference in
electronegativities is zero or less than 0.5, then the bond is pure covalent or
non-polar covalent bond. e.g. H–H, O=O, NºN, NCl3,
CH4, CH3–CH3, H2C=CH2,
F–F, Cl–Cl, I–I, Br–Br, etc. If the difference in electronegativities is more
than 0.5 but less than 1.7, then the bond is polar covalent or partially ionic
in character e.g. H –F , H – Cl , H – O – H etc.
Types
of Covalent Bond According to No of Shared Pairs
Covalent
bond is of three types:
(i) Single Covalent Bond.
(ii) Double
Covalent Bond.
(iii) Triple
Covalent Bond.
1. Single Covalent
Bond
Such a covalent bond formed by the sharing of
only one pair of electrons (i.e. 2
electrons) between bonded atoms each contributing one electron is called Single
Covalent Bond and is denoted by single
short line (–) between bonded atoms. e.g. H–H, F–F, Cl–Cl,
Br–Br, I–I, H–Cl, H–Br, H–I, H–F, H2O, H2S, NH3,
PH3, PCl3, NCl3, CH4, CCl4,
CH3Cl, CH3–CH3, AlCl3, BCl3,
BF3, FeCl3 etc. The electronic formulae or Lewis Formulae
or Cross-dot formulae of some compounds containing single bond are given
below:-
2. Double Covalent
Bond
Such a covalent bond formed by the sharing of
only two pairs of electrons (i.e. 4
electrons) between bonded atoms each contributing two electrons is called
Double Covalent Bond and is represented by two
short line (=) between bonded atoms e.g. O=O, O=C=O (CO2), H2C=CH2
(C2H4), S=C=S (CS2), SO2, SO3,
NO2.
3. Triple Covalent
Bond
Such a covalent bond formed by the sharing of
only three pairs of electrons (i.e.
6 electrons) between bonded atoms each contributing three electrons is called
Triple Covalent Bond and is denoted by three
short lines (º) between bonded
atoms. e.g. NºN (N2), HCºCH
(C2H2), H–CºN (HCN).
Characteristics
of Ionic Compounds
The compounds in which oppositely charged
ions (i.e. charged atoms) are bounded by electrovalent or ionic bonds are
called Ionic or Electrovalent Compounds. They have following common properties:
1. Strong Binding
forces, Physical State
In ionic compounds, it is improper to say
that any two ions are bonded to each other to produce single separate
(discrete) ionic molecules rather in ionic compounds, the large numbers of
oppositely charged ions are mutually surrounded by each other in an alternating
cation-anion pattern and arranged systematically in an orderly
three-dimensional arrangement that extends in all directions to form giant solid
aggregates called crystal lattice in which opposite ions are tightly held
together by strong electrostatic forces. Because of strong binding forces,
ionic compounds are always hard and rigid crystalline solids at room
temperature. (However they are brittle and easily broken when a stress is
applied on them.) They are never liquids or gases under normal conditions.
2. High Melting
and Boiling Points and Non-Volatile Nature
They are non-volatile in nature having high
melting and boiling points due to the strong interionic binding forces in the
form of electrostatic forces. e.g. NaCl has high melting point of 801°C.
3. Conductivity in
molten or solution
state
They do not conduct electricity are
non-conductor (i.e. non-electrolyte) in the solid state as the ions are not
free to move. But they are electrolyte (good conductor of electricity in
molten state or in solution form) due to free movement of the ions. This is
because in the presence of polar solvents the interionic binding forces become
so weak that ions are separated and start moving freely under the influence of
electric current.
4. Solubility in polar Solvents
They are insoluble in non-polar or organic
solvents of low dielectric constant like benzene, ether, carbon tetrachloride
etc. because there is no attraction between the ions of ionic compounds and the
molecules of non-polar solvents.
Ionic compounds are soluble in polar solvents
like water of high dielectric constants as like dissolves like.
5. Nature of
Reactions
They undergo ionic
reactions in solution which are fast and instantaneous and catalyst
independent.
6. Nature of Bonds
(a).Composed of Ions
Ionic compounds do not contain molecules.
They consist of aggregates of ions.
(b).Non-rigid and Non-directional
(c).No Geometry and no Isomerism
Characteristics
of Covalent Compounds
The compounds in which atoms are bounded by
covalent bond comprising of molecules are called covalent compounds. They have
following common properties:
1. Physical State
Covalent compounds are usually made up of
discrete molecules (having definite shape because of directional nature of
covalent bond) with weak intermolecular forces. Because of weak intermolecular
forces, covalent compounds are often gases or volatile liquids and soft low
melting solids (with separate layered molecules). In few cases, three
dimensional giant covalent structures are formed rather than discrete units.
Such giant network covalent solids are very hard showing high melting and
boiling points.e.g. Diamond, graphite, silica (SiO2), silicates etc.
2. Volatile
in Nature
They are volatile in nature having low
melting and boiling points because of the weak intermolecular binding forces
except the giant covalent molecules.
3. Non-Conductivity
They are insulators or non-conductors i.e. do
not conduct electricity (except graphite and aqueous solutions of acids) as
they are devoid of ions.
4. Solubility
in Organic (Non-polar) Solvents
They are insoluble in polar solvents like
water but soluble in non-polar or organic solvents of low dielectric constant
such as benzene, ether, carbon tetrachloride etc. as like dissolves like.
5. Composed
of Ions
Covalent compounds do not contain ions. They
consist of discrete units molecules.
No comments:
Post a Comment