Ionic Bond
Ionic Bond
** This type of
bond is established by transfer of an electron from one atom to another.
** Let us consider
a general case when an atom A has one electron in the valence shell and another
atom B has seven electrons. A has one electron in excess and B has one electron
short than the stable octet. Therefore, A transfers an electron to B and in
this transaction both the atoms acquire a stable electronoctet.
** The
resulting positive ion (cation) and negative ion (anion) are held together by
electrostatic
attraction.
** The
electrostatic attraction between the cation (+) and anion (–) produced by
electron transfer constitutes an Ionic or Electrovalent bond.
** The
compounds containing such a bond are referred to as Ionic or Electrovalent Compounds.
Conditions for formation of Ionic Bond
The conditions
favourable for the formation of an ionic bond are:
(1) Number of valence electrons
** The atom A
should possess 1, 2 or 3 valence electrons, while the atom B should have 5, 6
or 7 valence electrons.
** The elements
of group IA, IIA and IIIA satisfy this condition for atom A and those of groups
VA, VIA, and VIIA satisfy this condition for atom B.
(2) Net lowering of Energy
** To form a
stable ionic compound, there must be a net lowering of the energy. In other
words energy must be released as a result of the electron transfer and
formation of ionic compound by the following steps :
(a) The
removal of electron from atom A (A – e– → A+) requires
input of energy, which is the ionization energy (IE). It should be low.
(b) The
addition of an electron to B (B + e– →B– ) releases
energy, which is the electron affinity of B (EA). It should be high.
(c) The
electrostatic attraction between A+ and B– in the solid
compound releases energy, which is the electrical energy. It should also be
high.
** If the energy
released in steps (b) and (c) is greater than the energy consumed in step (a),
the overall process of electron transfer and formation of ionic compound
results in a net release of energy. Therefore, ionization of A will occur and
the ionic bond will be formed. For example, in case of formation of sodium chloride
(NaCl), we have:
** The net
energy released is 187 + 85 – 119 = 153 kcal. Since the overall process results
in a lowering of energy, the ionic bond between Na and Cl will be formed.
(3) Electronegativity difference of A and B
** From the
line of argument used in (2), we can say that atoms A and B if they have
greatly different electronegativities, only then they will form an ionic bond.
** In fact, a
difference of 2 or more is necessary for the formation of an ionic bond between
atoms A and B.
** Thus Na has electronegativity
0.9, while Cl has 3.0. Since the difference is (3.0 – 0.9) = 2.1, Na and Cl
will form an ionic bond.
Factors governing the formation of Ionic bond
(1) Ionization Energy
** The ionization
energy of the metal atom which looses electron(s) should be low so that the
formation of
+vely charged ion is easier.
** Lower the ionization
energy greater will be the tendency of the metal atom of change into Cation and
hence greater will be the ease of formation of ionic bond. That is why alkali
metals and alkaline earth metals form ionic bonds easily.
** Out of these
two, alkali metals form ionic bonds easily as compared to alkaline earth metals.
** In a group
the ionization energy decreases as we move down the group and therefore, the
tendency to form ionic bond increases in a group downward. Due to this reason
Cs is the most electropositive atom among the alkali metals.
(2) Electron Affinity
** The atom
which accepts the electron and changes into anion should have high electron
affinity.
** Higher the
electron affinity more is the energy released and stable will be the anion
formed.
** The elements
of group VI A and VII A have, in general, higher electron affinity and have
high tendency to form ionic bonds. Out of these two, the elements of group VII
A (halogens) are more prone to the formation of ionic bond than the elements of
group VI A. In moving down a group the electron affinity decreases and,
therefore, the tendency to form ionic bond also decreases.
(3) Lattice Energy
** After the
formation of cations and anions separately, they combine to form ionic
compound.
** In this
process, energy is released. It is called Lattice Energy. It may be defined as
“the amount of energy released when one mole of an ionic compound is formed
from its cations and anions.”
** Greater the
lattice energy, greater the strength of ionic bond. The value of lattice energy
depends upon the following two factors:
(a) Size of the
ions
In order to
have the greater force of attraction between the cations and anions their size
should be small as the force of attraction is inversely proportional to the
square of the distance between them.
(b) Charge on
Ions
** Greater the
charge on ions greater will be the force of attraction between them and, therefore,
greater will be the strength of the ionic bond.
** Necessary
for the formation of an ionic bond between atoms A and B. Thus Na has electronegativity
0.9, while Cl has 3.0. Since the difference is (3.0 – 0.9) = 2.1, Na and Cl
will form an ionic bond.
Some examples of Ionic compounds
Here we will
discuss the formation of Lewis formula or Electron dot formula of some binary ionic
compounds, for illustration.
Sodium Chloride, NaCl
** A simple
sodium chloride molecule is formed from an atom of sodium (Na) and one atom of
chlorine (Cl).
** Na (2, 8, 1)
has one valence electron, while Cl (2, 8, 7) has seven. Na transfers its
valence electron to Cl, and both achieve stable electron octet. Thus Na gives
Na+ and Cl gives Cl– ion, and the two are joined by an
ionic bond.
** Ionic
Compounds Exist as Crystals. The (+) and (–) ions attract each other with
electrostatic force that extends in all directions. This means that ions will
be bonded to a number of oppositely charged ions around them. Therefore in
solid state, single ionic molecules do not exist as such.
** Rather many
(+) and (–) ions are arranged systematically in an alternating cation-anion pattern
called the crystal lattice.
** The crystal
lattice of NaCl is shown Figure. It will be noticed that here a large number of
Na+ and Cl– ions are arranged in an orderly fashion so as
to form a cubic crystal. Each Na+ ion is surrounded by 6 Cl– ions
and each Cl– ion is surrounded by 6 Na+ ions. This makes
a network of Na+ and Cl– ions which are tightly held together
by electrostatic forces between them.
** Although
discrete molecules Na+Cl– do not exist in the solid form
of ionic compounds, independent molecules do exist in the vapour form of such
compounds.
Magnesium Chloride MgCl2
** Magnesium
(Mg) has two valence electrons, while chlorine (Cl) has seven.
** The
magnesium atom transfers its two electrons, one to each chlorine atom, and thus
all the three atoms achieve the stable octet.
** In this way
Mg atom gives Mg2+ ion and the two Cl atoms give 2Cl1–,
forming Mg+2 Cl21- (or MgCl2).
Calcium Oxide CaO
** Calcium (Ca)
has two valence electrons, while oxygen (O) has six. Calcium atom transfers its
two valence electrons to the same oxygen atom.
** Thus both Ca
and O achieve the stable electron-octet, forming Ca2+ and O2–
ions. Thus is obtained the molecule of calcium oxide, Ca2+O2–.
Aluminium Oxide Al2O3
** Here the
aluminium atom (Al) has three electrons in the valence shell (2, 8, 3), while
oxygen has six (2, 6).
** Two atoms of
aluminium transfer their six electrons to three oxygen atoms. Thus are the
electronoctets of the two Al atoms and three O atoms achieved. The two Al atoms
deprived of three electrons each, give 2Al3+ ions, while the three O
atoms having gained two electrons each give 3O2– ions.
** In this way,
we get Al2 3+ O3 2-
or Al2O3.
Characteristics of Ionic compounds
The ionic
compounds are made of (+) and (–) ions held by electrostatic forces in a
crystal lattice. Each ion is surrounded by the opposite ions in alternate positions
in a definite order in all directions. This explains the common properties of
ionic compounds.
(1) Solids at
Room Temperature
On account of
strong electrostatic forces between the opposite ions, these ions are locked in
their allotted
positions in the crystal lattice. Since they lack the freedom of movement
characteristic of the liquid state, they are solids at room temperature.
(2) High
Melting Points
** Ionic compounds
have high melting points (or boiling points). Since the (+) and (–) ions are tightly
held in their positions in the lattice, only at high temperature do the ions
acquire sufficient kinetic energy to overcome their attractive forces and
attain the freedom of movement as in a liquid.
** Thus ionic
compounds need heating to high temperatures before melting.
(3) Hard and
brittle
** The crystals
of ionic substances are hard and brittle. Their hardness is due to the strong electrostatic
forces which hold each ion in its allotted position. These crystals are made of
layers of (+) and (–) ions in alternate positions so that the opposite ions in
the various parallel layers lie over each other.
** When
external force is applied to a layer of ions , with respect to the next, even a
slight shift brings the like ions in front of each other. The (+) and (–) ions
in the two layers thus repel each other and fall apart. The crystal cleaves
here.
(4) Soluble in
water
** When a
crystal of an ionic substance is placed in water, the polar water molecules
detach the (+) and (–) ions from the crystal lattice by their electrostatic
pull. These ions then get surrounded by water molecules and can lead an
independent existence and are thus dissolved in water.
** By the same reason,
non-polar solvents like benzene (C6H6) and hexane (C6H14)
will not dissolve ionic compounds.
(5) Conductors
of electricity
** Solid ionic
compounds are poor conductors of electricity because the ions are fixed rigidly
in their positions.
** In the
molten state and in water solutions, ions are rendered free to move about. Thus
molten ionic compounds or their aqueous solutions conduct a current when placed
in an electrolytic cell.
(6) Do not
exhibit isomerism
The ionic bond
involving electrostatic lines of force between opposite ions, is non-rigid and nondirectional.
The ionic compounds, therefore, are incapable of exhibiting
stereoisomerism.
(7) Ionic
reactions are fast
Ionic compounds
give reactions between ions and these are very fast.
Comparison of Ionic and covalent bonds
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.
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