IONIC BOND OR ELECTROVALENT BOND
Ionic bond is formed by the transference of one or more electrons from one atom to the other. This type of bond usually comes into existence between an electropositive (generally metal) and electronegative (generally non-metal) atom. The metal atom loses its valence electrons and changes into a positive ion (cation) while a non-metallic atom gains electrons and gets converted into negative ion (anion). They do so in order to become stable by attaining 8 (or 2) electrons in the outermost shell. The oppositely charged ions are held together by electrostatic force of attraction. Thus, ionic bond may be defined as the electrostatic force of attraction which holds the positively charged (cation) and negatively charged ( anion species together.
Let us study the formation of sodium chloride to understand the ionic bond.
Formation of Sodium Chloride (NaCI)
Sodium atom, Na (2, 8, 1) has only 1 electron in its valence shell. In order to acquire the configuration of nearest noble gas Ne (2, it has to lose its valence electron, Chlorine atom, Cl (2, 8, 7), on the other hand, has seven electrons in its valence shell. It can acquire the configuration of nearest noble gas, Ar (2, 8, by· gaining one electron. Thus, sodium atom transfers its valence electron to chlorine atom resulting in the formation of sodium ion (Na+) and chloride ion (CI-) respectively. The electrostatic force of attraction holds these oppositely charged ions together. Let us represent the above changes by using Lewis Symbols.
Fig. 7.1. Ionic bond formation for sodium chloride.
Formation of Calcium Oxide (CaO)
Electronic configuration of calcium atom is 2, 8, 8, 2 whereas that of oxygen atom, it is 2, 6, calcium tends to acquire the configuration of Ar (2, 8, by losing 2 valence electrons and forming a divalent cation Ca2+. Oxygen atom gains 2 electrons to form oxide ion (02-) and thus, it attains the configuration of Ne (2, 8, 8). The Ca2+ ion and oz.- ions are held by the electrovalent bond as shown in Fig. 7 .2.
Fig. 7.2. Formation of ca1c1um ox1de (CaO).
Formation Magnesium Chloride (Mg Cl2))
In the formation of magnesium chloride, Mg atom (2, 8, 2) loses its two valence electrons to form divalent Mg2+ ion. These two electrons are gained by two chlorine atoms (one each) to form chloride ions. The oppositely charged ions are held by ionic bonds as shown in Fig. 7 .3.
Fig. 7 .3. Formation of magnesium chloride (MgCI2).
It may be noted in the ionic bond model, the ions are considered to be spherical charged particles which have a uniform field of influence around them, hence, ionic bond is non-directional in nature.
It is the number of electrons which an atom loses or gains while forming an ionic bond. The atom which loses electrons is called electropositive atom while the atom which gains electrons is called electronegative atom. The positive ( +) sign is put before the valency of electropositive atom while negative(-) sign is put before the valency of electronegative atom. For example, electrovalency of Na is + 1; Ca is + 2 and – that of Cl is -1. In short, electrovalency is equal to the unit charges on the ion.