Ionic (or)
Electrovalent bond
The electrostatic attraction force existing between the cation and the
anion produced by the electron transfer from one atom to the other is known as
the ionic (or) electrovalent bond. The compounds containing such a bond are
referred to as ionic (or) electrovalent compounds.
Ionic bond is non directional and extends in all directions. Therefore,
in solid state single ionic molecules do not exist as such. Only a network of
cations and anions which are tightly held together by electro-static forces
exist in the ionic solids. To form a stable ionic compound there must be a net
lowering of energy. That is, energy is released as a result of electovalent
bond formation between positive and negative ions.
When the electronegativity difference between the interacting atoms are
greatly different they will form an ionic bond. In fact, a difference of 2 or
more is necessary for the formation of an ionic bond. Na has electronegativity
0.9 while Cl has 3.0, thus Na and Cl atoms when brought together will form an
ionic bond.
For example, NaCl is formed by the electron ionisation of sodium atom to
Na+ ion due to its low ionisation potential value and chlorine atom
to chloride ion by capturing the odd electron due to high electron affinity.
Thus, NaCl (ionic compound) is formed. In NaCl, both the atoms possess unit
charges.
i. Na(g) '2s22p63s1' --- (ionization) -- > Na+(g) + e-
'2s2sp6'
ii. Cl(g) + e- '3s23p5' --- (Affinity) --
> Cl- '3s2, 3p6
'
iii. Na+ + Cl- --- (electrostatic attraction)--- > NaCl
Na+ : Sodium ion Cl-
: Chloride ion
NaCl : ionic/crystalline compound is formed
In CaO, which is an ionic compound, the formation of the ionic bond
involves two electron transfers from Ca to O atoms. Thus, doubly charged
positive and negative ions are formed.
Ca --- ionization
--- > Ca2+ + 2e- (Calcium Cation)
3p64s2
O + 2e-
----- electron aaffinity -àO2- (Oxide anion)
2s22p4
2s22p6
Ca2+ + O2- ---- electrostatic attraction --- > CaO
(ionic compound)
Ionic bond may be also formed between a doubly charged positive ion with
single negatively charged ion and vice versa. The molecule as a whole remains
electrically neutral. For example in MgF2, Mg has two positive charges and each
fluorine atom has a single negative charge. Hence, Mg2+ binds with
two fluoride (F-) ions to form MgF2 which is electrically
neutral.
Mg -- ---
----- > Mg2++ 2e-
2s2 2p6 3s2 2s2 2p6
2e + 2F ---- -----
---- > 2F-
2s22p5 2s22p6
Mg2+ + 2F- --- > MgF2 Magnesium - fluoride (an ionic compound)
Similarly in Aluminium bromide (AlBr3),
Aluminium ion has three positive charges and therefore it bonds with three
Bromide ions to form AlBr3 which is a neutral ionic molecule.
Al ---- ---- > Al3+
+ 3e-
2p6 3s23p1 2s2,2p6
3Br + 3e- ---- ---- > 3Br-
4s24p5 4s24p6
Al3+ + 3Br- --- --- > AlBr3 (ionic bond )
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