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)
O + 2e- ----- electron aaffinity -àO2- (Oxide anion)
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-
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-
Al3+ + 3Br- --- --- > AlBr3 (ionic bond )