In this article, we shall study the properties of ionic compounds.

Physical State:

Due to strong electrostatic force present between the oppositely charged ions, they are held closer and fixed at specified positions in the crystal lattice. Hence ionic compounds usually exist in the form of crystalline solids at room temperature. Actually, these compounds do not possess a molecule but it is a well-defined geometric arrangement of positive and negative ions in the crystal lattice.

In sodium chloride (NaCl), each Na⁺ ion is surrounded by 6 Cl^– ions and each Cl^– ion is surrounded by 6 Na⁺ ions. The arrangement is as shown below.

High Melting and Boiling Points of Ionic Compounds:

Due to strong electrostatic force present between the oppositely charged ions, they are held closer and fixed at specified positions in the crystal lattice. Hence very high temperature is required to separate from each other and to move freely as in the case of liquid. Hence the boiling and melting points of ionic compounds are generally very high.

Hard and Brittle Nature of Ionic Compounds:

Due to strong electrostatic force present between the oppositely charged ions, they are held closer and fixed at specified positions in the crystal lattice. There are layers of positive ions and negative ions. When force is applied to them the layer may slide and the ions having similar charges may come near each other. In such a case, the two layers repel each other and the crystal gets cleaved. Hence ionic solids are brittle.

Electrical Conductivity:

Due to strong electrostatic force present between the oppositely charged ions, they are held closer and fixed at specified positions in the crystal lattice. Hence in solid-state ionic compounds do not conduct electricity. In the Fused state or the dissolved state, the ions of the ionic compound can move from one position to another freely. Hence in fused state, ionic compounds conduct electricity.

Lattice Energy (Δ_latticeH):

The enthalpy change involved in the formation of one mole of an ionic crystal from its constituent gaseous positive and negative ion is called lattice energy. The formation of an ionic compound is an exothermic process. Hence the enthalpy change involved in the process of formation of an ionic compound is negative. The lattice energy decreases with the increase in the ionic size.

Hydration Energy (Δ_hydrationH|):

The enthalpy change involved in the hydration of one mole of gaseous ions of each type of an ionic compound (solid) is called hydration energy. Hydration of ionic compounds is exothermic. Hence the enthalpy change involved in the process of hydration of an ionic compound is negative. The hydration energy decreases with the increase in the ionic size.

Solubility in water:

Water has a high dielectric constant hence it weakens the electrostatic force between the ions of an ionic compound. Due to which the ions get separated and get surrounded by the water molecule. This dissolves the ionic compound in water. Hence ionic compounds are soluble in water.

Organic solvents like benzene, ether, hexane have low dielectric constant and are unable to weaken the electrostatic force between the ions of the ionic compound. Thus they are unable to separate the ions. Hence ionic compounds are not soluble in organic solvents.

The enthalpy of solution of an ionic solid is numerically equal to the difference in its hydration energy and lattice energy

Mathematically,   Δ_solutionH  = Δ_hydrationH – Δ_latticeH

The Gibb’s energy (free energy) change is given by

ΔG  = ΔH – T ΔS    i.e.

Δ_solutionG  = Δ_solutionH – T Δ_solutionS

Hence for a compound to get dissolved in water, Gibb’s energy (free energy) change involved in the solution formation process is negative. Thus the quantity (Δ_solutionH – T Δ_solutionS) is negative. The quantity Δ_solutionS is positive i.e. the quantity T Δ_solutionS is positive. It means the quantity Δ_solutionH should be smaller than T Δ_solutionS. Hence a more negative value of Δ_solutionH will help the greater dissolution of an ionic compound in water.

Hence we can conclude that

• If Δ_hydrationH > Δ_latticeH, the salt would dissolve in water.
• If Δ_hydrationH < Δ_latticeH, ordinarily the salt would dissolve in water. It will start dissolving when Δ_solutionH < T Δ_solutionS

Stereoisomerism:

Isomers having the same molecular formula, same structural formula but different configurations are called stereo Isomers and the phenomenon is known as stereoisomerism. The ionic compounds are neither rigid or directional hence they do not show stereoisomerism.

Ionic Reactions:

The ionic reaction involves the mutual interaction of ions. Due to the presence of ions,  the reactions of ionic compounds are very fast

Born-Haber Cycle

The energy change involved in the formation of ionic bonds from the constituent elements can be represented by a cycle called Born-Haber Cycle. It uses Hess’s law of thermodynamics to calculate the change in enthalpy during ionic bond formation.

Let us consider the formation of 1 mole of sodium chloride from sodium and chlorine.

The Born-Haber Cycle can be simply stated as Heat of formation of ionic crystal = Heat of atomization + Dissociation energy+ (sum of Ionization energies)+ (sum of Electron affinities) +  Lattice energy.

If energy is released (exothermic reaction), put a negative sign in front of the value of the energy; if energy is absorbed (endothermic reaction), the value of energy should be positive.

Note:

In this general equation, the electron affinity is added. However, when plugging in a value, determine whether energy is released (exothermic reaction) or absorbed (endothermic reaction) for each electron affinity.

Science > Chemistry > Physical Chemistry > Nature of Chemical Bond > Properties of Ionic Compounds

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In this article, we shall study the factors affecting the formation of the ionic bond and the concept of variable electrovalency.

Ionization energy of Electropositive Atom:

It is defined as the amount of energy required to remove the most loosely bound electron from an isolated gaseous atom of an element. The ionization energy of an atom can be considered as a measure to lose an electron and form a cation. The lesser the ionization energy, the greater is the ease of the formation of a cation and thus it can form ionic bond easily.

Alkali metals and alkaline earth metals have low ionization energy hence they have a tendency to form ionic compounds.

Electron Affinity or Electron Gain Enthalpy of Electronegative Atom:

It is defined as the amount of energy released when an electron is added to an isolated gaseous atom of an element. The electron gain enthalpy of an atom can be considered as a measure to gain an electron and form an anion. The higher the energy released during this process, the easier will be the formation of an anion.

Elements of groups 16 and 17 have more negative values of electron gain enthalpies hence they have a tendency to form ionic compounds. 

Thus, the low ionization energy of a metal atom and high electron affinity of a non-metal atom facilitate the formation of an ionic bond between them.

Lattice energy or Lattice Enthalpy:

The enthalpy change involved in the formation of one mole of an ionic crystal from its constituent gaseous positive and negative ion is called lattice energy. The formation of an ionic compound is an exothermic process. Hence the enthalpy change involved in the process of formation of an ionic compound is negative.

Higher the lattice energy, the greater is the tendency of the formation of an ionic bond. The higher the charges on the ions and the smaller the distance between them, the greater is the force of attraction between them.

The lattice energy depends on the following factors

• Size of the atom: If the ions formed are smaller in size, the inter-nuclear distance is less and hence the inter-ionic attractive force is greater. Hence the lattice energy will be high. Thus smaller the size of ions, greater is the lattice energy.
• Charge on the ions: Higher the charges on the ion, greater the inter-ionic attractive force. Hence lattice energy is high. Thus higher the charge on the ions, greater is the lattice energy.

Difference in Electronegativities:

The tendency of an atom to attract the bonding or a shared pair of electrons towards its own side in a covalent bond is called electronegativity of that atom. Higher the difference in electronegativities of the two atoms, greater will be the ease to form an ionic bond.

A difference of 2 units (pauling) in electronegativities is required to form an ionic bond.

The Concept of Variable Electrovalency:

There are many elements in the periodic table which are capable of forming more than one type of ions having different charges. Thus they possess more than one electrovalency. This phenomenon is known as a variable electrovalency. For example, copper forms cuprous (Cu⁺) and cupric (Cu ²⁺) ions. Iron forms ferrous (Fe²⁺) and ferric (Fe³⁺) ions.

Causes of Variable Electrovalency:

Unstable Nature of Core (Kernel):

When an atom loses one or more electrons, a cation is formed. The remaining part of the atom left is called a core or a kernel of the atom.

During the formation of positive ion-neutral atom loses one or more (definite) number of electrons to form a cation. If core or kernel form is stable then it will exhibit definite electrovalency.

Na               →      Na⁺       +     e^–

1s² 2s² 2p⁶ 3s¹          1s² 2s² 2p⁶

Sodium atom   core or kernel of sodium

If core or kernel of ion formed is not stable then to acquire greater stability it is compelled to lose more electrons. This fact gives rise to variable electrovalency. Let us consider the case of iron Fe (Z = 26)

Fe                   →          Fe ²⁺       +    2 e^–

1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²       1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶

(2, 8, 14, 2)                          (2, 8, 14)  less stable

Fe²⁺               →      Fe ³⁺       +     e^–

1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶        1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵

(2, 8, 14)                          (2, 8, 13)

more stable  due to exactly half filled d orbitals

Inert Electron Pair Effect:

The reluctance of ns² electron pair to get excited and to take part in bond formation is called inert electron pair effect.

Let us consider case of tin Sn (Z = 50). Electronic configuration of tin is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p². (2, 8, 18, 18, 4). It has 4 electrons and can form Sn⁴⁺ ion by donating 4 electrons and acquires configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ (2, 8, 18, 18).

But due to reluctance of 5s² electron pair to get excited and to take part in bond formation, it loses only two electrons and acquires configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ (2, 8, 18, 2) an forms Sn²⁺ ion.

Thus tin shows two electrovalencies due to the inert pair effect.

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