2.1 IONIC BONDING

Ionic bonding is the complete transfer of valence electron(s) between atoms. Ionic bonds need an electron donor, typically a metal, and an electron acceptor, a non-metal. It’s a kind of attraction that generates two oppositely charged ions. In ionic bonds, the metal loses electrons to become a charged ion, whereas the non-metal accepts those electrons to become a charged ion.
Ionic bonding is ascertained as a result of metals which have few electrons in their outermost orbital. By losing those electrons, these metals are able to attain inert gas configuration and satisfy the octet rule. Similarly, non-metals that have near eight electrons in their valence shells tend to settle for electrons to attain inert gas configuration. In ionic bonds, charge of the compound should be zero.
The most common example of ionic bonding is the formation of sodium chloride wherein an atom of sodium combines with a chlorine atom. The electronic configurations of Sodium (Na) is 2, 8, 1 and that of Chlorine (Cl) is 2, 8, 7.

From the configuration it is evident that an atom of chlorine requires only one electron to attain the configuration of the nearest noble gas, i.e. Argon (2, 8, 8) while an atom of sodium requires to lose the single electron in its outermost shell to acquire the configuration of the nearest noble gas, i.e. Neon (2, 8). In such a scenario, the sodium atom donates its outermost electron to the chlorine atom, which requires only one electron to attain octet configuration. The sodium ion becomes positively charged due to the loss of an electron, whereas the chloride ion becomes negatively charged due to gain of an additional electron. The oppositely charged ions, thus formed, are attracted to each other and result in the formation of an ionic bond.

2.1.1 Characteristics

Presence of ionic bonds influences the chemical and physical properties of the resulting compounds. The prominent characteristics of ionic bonds are enlisted below:

• Due to the fact that metals tend to lose electrons and non-metals tend to gain electrons, ionic bonding is common between metals and nonmetals.
• While naming these compounds, the name of the metal always comes first and the name of the non-metal comes second. For instance, in case of sodium chloride (NaCl), sodium is the metal, whereas chlorine is the non-metal.
• Compounds that contain ionic bonds readily dissolve in water as well as several other polar solvents. The solubility in polar solvents like water may be explained by the dipole nature of water. Water molecules pull the ions of the ionic compound from the lattice. These ions then encircled by water dipoles with the oppositely charged ends directed towards them. The electrovalent compound dissolves in the solvent if the value of the solvation energy is higher than the lattice energy of that compound. The value of solvation energy depends on the relative size of the ions. Smaller the ions more is the solvation. The non-polar solvents do not solvate ions and thus do not release energy due to which they do not dissolve ionic compounds.
• Solid ionic compounds do not conduct electricity because their constituent ions are fixed in their positions. But, when melted or dissolved in water, they conduct electricity because the ions become free in a solution.
• Ionic compounds tend to have higher melting points; therefore we can say that these bonds remain stable for a greater temperature range.
• Ionic compounds are typically crystalline in nature with constituent units as ions. Force of attraction between the ions is non-directional and extends to all directions. Every particle is surrounded by a variety of oppositely charged ions and this is termed as coordination number. Since ionic forces of attraction act on all the directions, therefore, the ionic compounds don’t posses directional characteristic and therefore don’t show stereoisomerism.

2.1.2 Energy Considerations Ionic Bonding

The formation of ionic bond can be considered to proceed in three steps:

(i) Formation of gaseous cations: The energy required for this step is called ionization energy (IE).

A(g) + IE → A+ (g) + e–

(ii) Formation of gaseous anions: The energy released from this step is called electron affinity (EA).

X(g) + e– → X–(g) + EA

(iii) Packing of ions of opposite charges to form ionic solid: The energy released in this step is called lattice energy.

A+(g) + X–(g) → AX(s) + energy

As mentioned in the previous section, for stable ionic bond formation the total energy released should be more than the energy required. The factors which influence have been discussed below:

Ionization energy: In the formation of ionic bond a metal atom loses electron to form cation. This process requires energy equal to the ionization energy. Lesser the value of ionization energy, greater is the tendency of the atom to form cation. For example, alkali metals form cations quite easily because of the low values of ionization energies. We can thus say these metals are more susceptible to form ionic bonds.

Electron affinity: Electron affinity is the energy released when gaseous atom accepts electron to form a negative ion. Thus, the value of electron affinity gives the tendency of an atom to form anion. Greater the value of electron affinity more is the tendency of an atom to form anion. For example, halogens having highest electron affinities within their respective periods to form ionic compounds with metals very easily.

Lattice energy: Once the gaseous ions are formed, the ions of opposite charges come close together and pack up three dimensionally in a definite geometric pattern to form ionic crystal.

Since the packing of ions of opposite charges takes place as a result of attractive force between them, the process is accompanied with the release of energy referred to as lattice energy. Lattice energy may be defined as the amount of energy released when one mole of ionic solid is formed by the close packing of gaseous ion.
We can thus coin the conditions for the stable ionic bond as:

• IE of cation forming atom should be low
• EA of anion forming atom should be high
• Lattice energy should be high

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