Ionic Bond

Ionic or electrovalent bonds form when two or more atoms complete the transfer of one or more electrons from one atom to another. This process involves an electropositive (metal) atom losing electrons to become a cation, and an electronegative (non-metal) atom gaining electrons to become an anion. 

The resulting attraction between the oppositely charged ions forms the ionic bond or electrovalent bond, which is characterized by the electrostatic force of attraction between the cation and anion.

1.0Definition and Formation of Ionic Bonds

Ionic bonds are electrostatic attractions between positively charged ions (cations) and negatively charged ions (anions) that result from the transfer of electrons.

Formation: Typically occurs between atoms with a large difference in electronegativity, where one atom (usually a metal) loses electrons to become a positively charged ion, and another atom (usually a nonmetal) gains those electrons to become a negatively charged ion.

                             M(g)  → M⁺(g) + e^ー

                      X(g)  + e^ー →  X^ー

2.0Conditions for Forming Ionic Bonds

Several factors influence the formation of ionic bonds, which are bonds formed between ions of opposite charges. Factors affecting the formation of Ionic bond –

1. Ionization Energy (I.E.)

Ionization energy (IE) is the amount of energy required to remove an electron from the outermost orbit of an isolated gaseous atom to form a positive ion or cation. This process involves energy absorption.

Lesser Ionisation energy → Greater tendency to form cation.

Ionic bonds typically form more easily between elements with low ionization energies and those with high negative electron affinities. This combination promotes the efficient transfer of electrons from one atom to another, forming strong electrostatic attractions between the resulting ions. Such conditions lead to the formation of ionic compounds, characterized by their high melting and boiling points.

• Factors Influencing IE: 

The magnitude of ionization energy depends on atomic size, nuclear charge, and the electron configuration of the atom. Atoms with a larger nuclear charge and smaller atomic radii tend to have higher ionization energies because the electrons are more strongly attracted to the nucleus.

• Periodic Trends: 

Ionization energy generally increases across a period (from left to right) in the periodic table due to increasing nuclear charge and decreases down a group as atomic size increases and electrons are further from the nucleus.

• Multiple Ionization Energies: 

There are successive ionization energies for removing more than one electron. Each successive ionization energy is higher than the last, as it becomes more difficult to remove an electron from a positively charged ion.

2. Electron Affinity (E.A.)

Electron affinity is the amount of energy released when an electron is added to an isolated gaseous atom to form a negative ion (anion). This process involves energy release.

Higher electron affinity → Greater tendency to form anion

• Trends in Electron Affinity: 

Electron affinity can vary significantly across the periodic table. It generally increases across a period due to the atoms becoming more eager to complete their valence shells. However, there are exceptions, such as the elements in group 2 and group 18, which have lower or negative electron affinities due to stable electron configurations.

• Influence of Electronic Configuration: 

Atoms with nearly full or half-full p-orbitals (such as halogens) tend to have higher electron affinities because adding an electron completes a stable configuration, releasing more energy.

• Thermodynamic and Kinetic Factors: 

The electron affinity is also influenced by the atom's ability to stabilize additional electron density, which can be affected by both thermodynamic (energy considerations) and kinetic (probability of electron capture) factors.

3. Lattice Energy (U)

Lattice energy is a measure of the strength of the forces holding ions together in an ionic compound. It represents the energy released when gaseous cations and anions come together to form a solid ionic lattice.

• Factors Affecting Lattice Energy: 

The magnitude of lattice energy is influenced by the charge and size of the ions forming the lattice. Higher charges and smaller ionic radii lead to stronger attractions and thus higher lattice energies.

• Calculation of Lattice Energy:

Lattice energy can be estimated using the Born-Haber cycle, a thermochemical cycle that includes steps like ionization energy, electron affinity, and sublimation energy.

• Implications of Lattice Energy: 

Higher lattice energy indicates a more stable ionic compound and typically corresponds to higher melting and boiling points. It also affects the solubility and hardness of the ionic solid.

3.0Mechanism of formation of ionic compound or ionic crystal lattice (Born-Haber Cycle)

The Born-Haber Cycle is a thermodynamic cycle that helps to understand the formation of ionic compounds like sodium chloride (NaCl) from its constituent elements (sodium and chlorine) in their standard states. It provides a detailed pathway of energy changes involved in the formation of an ionic lattice, explaining why and how the ionic compounds are formed. Here's a step-by-step breakdown using the example of NaCl:

Steps in the Born-Haber Cycle for NaCl

1. Sublimation of Sodium (Na)

• Process: Solid sodium (Na) is converted into gas-phase sodium atoms.
• Energy Change: Endothermic (energy is absorbed).
• Equation: Na (s)→Na (g)

2. Ionization of Sodium (Na)

• Process: Gas-phase sodium atoms are ionized to form sodium ions by losing an electron.
• Energy Change: Endothermic (energy is absorbed).
• Equation: Na (g) → Na⁺(g)+e⁻

3. Dissociation of Chlorine Molecules (Cl₂)

• Process: Molecular chlorine (Cl₂) is broken into chlorine atoms.
• Energy Change: Endothermic (energy is absorbed).
• Equation: Cl2(g)→2Cl (g)

4. Electron Affinity of Chlorine

• Process: Chlorine atoms gain an electron to form chloride ions.
• Energy Change: Exothermic (energy is released).
• Equation: Cl (g)+ e⁻→Cl⁻(g)

5. Formation of Ionic Lattice

• Process: Sodium ions (Na⁺) and chloride ions (Cl^-) come together to form the solid ionic lattice of NaCl.
• Energy Change: Exothermic (energy is released).
• Equation: Na⁺(g) + Cl⁻(g)→ NaCl (s)

4.0Energy Diagram

• The Born-Haber Cycle can be represented in a cycle diagram, showing the sequence of steps starting from the elements in their standard states to the formation of the ionic compound. 
• The total energy released or absorbed in the cycle should equal the lattice energy of the ionic compound.

5.0Lattice Energy Calculation

Lattice Energy (U) is the net energy change when one mole of a solid ionic compound is formed from its gaseous ions. It can be calculated indirectly using the Born-Haber Cycle by considering all other energy changes in the cycle. The lattice energy is a critical factor in determining the stability and properties of the ionic compound.

Combine the energies from each step to solve for lattice energy using: 

Enthalpy of Formation (ΔH_f) = Sublimation Energy + $\frac{1}{2}$ Dissociation Energy + Ionization Energy − Electron Affinity − Lattice Energy

Rearranging to find lattice energy: 

Lattice Energy = Sublimation Energy + $\frac{1}{2}$ Dissociation Energy + Ionization    Energy − Electron Affinity − ΔH_f 

6.0Polarization (Fajan's Rule) and Covalent Nature in Ionic Compounds

Polarization occurs when a cation distorts the electron cloud of a nearby anion, causing the anion to become polarized. This distortion happens due to the electrostatic attraction between the cation and the anion's electron cloud, and the repulsion between the cation and the anion's nucleus.

• Polarizing Power: The ability of a cation to polarize an anion. It is also referred to as ionic potential or charge density.
• Polarizability: The tendency of an anion to get polarized by a cation. Increased polarization leads to greater sharing of electrons, imparting covalent character to an ionic bond.