Anand Classes provides comprehensive study material on Factors Affecting Formation of Ionic Bonds for Class 11 Chemistry. This detailed guide explains the key factors influencing the formation of ionic bonds, including ionization enthalpy, electron gain enthalpy, lattice energy, and electronegativity differences. The notes are designed to simplify complex concepts and help students excel in exams. Click the print button to download study material and notes.

Formation of Ionic Bond

The ease of formation of ionic compounds primarily depends upon:

• The ease of formation of positive and negative ions from the respective neutral atoms.
• The arrangement of positive and negative ions in the solid (called the lattice of the crystalline compound). This is also called the lattice of the crystalline compound.

Factors Affecting the Formation of Ionic Bonds

The tendency of atoms to form ionic bonds depends on the following factors:

1. Low Ionization Enthalpy

• The ease with which an atom can lose electrons depends upon its ionization enthalpy.
• Ionization enthalpy is the amount of energy required to remove the most loosely bound electron from an isolated gaseous atom to form a positive ion:

$$
\mathrm{M(g) \rightarrow M^{+}(g) + e^-} \quad \text{Ionization enthalpy}
$$

• Lower ionization enthalpy ⇒ greater ease of losing valence electrons.
• Therefore, in general, Metals with low ionization enthalpies (e.g., alkali and alkaline earth metals) therefore have a greater tendency to form ionic bonds.

2. High Electron Gain Enthalpy

• The tendency of an atom to gain electrons depends upon its electron gain enthalpy.
• Electron gain enthalpy is the energy released when an electron is added to an isolated gaseous atom to form a negative ion:

$$
\mathrm{X(g) + e^- \rightarrow X^{-}(g)} \quad \text{Electron gain enthalpy}
$$

• Higher (more negative) electron gain enthalpy ⇒ greater ease of anion formation.
• Therefore, in general, the elements which have high gain enthalpy values form ionic compounds.
• Non-metals like halogens and elements of the oxygen group typically have high electron gain enthalpies and readily form ionic compounds.

3. High Lattice Enthalpy

• The formation and strength of an ionic bond also depend upon the electrostatic force of attraction between oppositely charged ions.
• When free ions combine to form a crystal, energy is released.
• The energy released when 1 mole of a crystal is formed from free ions is called lattice enthalpy (U):

$$
\mathrm{M^{+}(g) + X^{-}(g) \rightarrow MX(s)} \:+\: \text{Lattice enthalpy (U)}
$$

Higher the value of lattice enthalpy, greater will be the ease of formation of the ionic compound.

• Higher lattice enthalpy ⇒ greater stability and easier formation of the ionic compound.

Condition for formation of ionic compounds using energy considerations

The formation of ionic compounds depends upon the total energy changes (Ionization enthalpy + Electron gain enthalpy + Lattice enthalpy) in the formation of ionic bonds.

• The Ionization process is endothermic (absorbs energy).
• Electron gain enthalpy may be exothermic (releases energy) or endothermic.
• Lattice enthalpy is always exothermic (releases energy).

If the net effect of these three processes results in overall energy release, an ionic bond will form.
This follows the principle that processes accompanied by a decrease in energy are spontaneous.

Example: Formation of NaCl

Consider the formation of NaCl:

1. Ionization of sodium
   $$\mathrm{Na(g) \rightarrow Na^{+}(g) + e^-} \quad \Delta i H = 495.8\ \mathrm{kJ\,mol^{-1}}$$
2. Electron gain by chlorine
   $$\mathrm{Cl(g) + e^- \rightarrow Cl^{-}(g)} \quad \Delta eg H = -348.7\ \mathrm{kJ\,mol^{-1}}$$
3. Lattice formation
   $$\mathrm{Na^{+}(g) + Cl^{-}(g) \rightarrow Na^{+}Cl^{-}(s)} \quad U = -788\ \mathrm{kJ\,mol^{-1}}$$

The energy released in steps (2) and (3) exceeds the energy absorbed in step (1).
Net energy released:

$$
\Delta H = -348.7 + (-788) + 495.8 = -640.9\ \mathrm{kJ\,mol^{-1}}
$$

Since the overall process releases 640.9 kJ mol⁻¹, NaCl is formed.

Thus, a quantitative measure of the stability of an ionic compound is provided by its lattice enthalpy and not simply by achieving octet of electrons around the ionic species in the gaseous state.

Summary of Conditions for Stable Ionic Bonding

Thus, to sum up, the conditions for stable ionic bonding are :

1. Low ionization enthalpy of the cation-forming atom.
2. Highly negative electron gain enthalpy of the anion-forming atom.
3. High lattice enthalpy to stabilize the crystal lattice.

Short Answer Conceptual Types Questions (SAT) on Formation of Ionic Bond

Q1. What is an ionic bond?

An ionic bond is the electrostatic force of attraction between positively charged cations and negatively charged anions formed by the transfer of electrons.

Q2. Why do metals form cations and non-metals form anions?

Metals have low ionization enthalpy, so they can easily lose electrons to form cations.
Non-metals have high (negative) electron gain enthalpy, so they readily gain electrons to form anions.

Q3. What role does lattice enthalpy play in ionic bond formation?

Lattice enthalpy (U) measures the energy released when gaseous ions combine to form a crystalline solid.
Higher lattice enthalpy ⇒ greater stability of the ionic compound.

Q4. Why is the formation of NaCl exothermic?

Although ionization of Na absorbs energy, the electron gain by Cl and lattice formation release much more energy, resulting in a net release of 640.9 kJ mol⁻¹.

Multiple Choice Questions (MCQs) With Answers and Explanation on Formation of Ionic Bond

1. Which of the following conditions favors ionic bond formation?
A) High ionization enthalpy of cation
B) Highly negative electron gain enthalpy of anion
C) Low lattice enthalpy
D) All of these

Answer: B (Highly negative electron gain enthalpy of the anion favors ionic bond formation.)

2. The energy released when 1 mole of an ionic crystal is formed from gaseous ions is called:
A) Ionization enthalpy
B) Electron gain enthalpy
C) Lattice enthalpy
D) Sublimation energy

Answer: C (Lattice enthalpy)

3. The formation of NaCl releases 640.9 kJ mol⁻¹ because:
A) Electron gain enthalpy of Cl is positive
B) Lattice energy released is very high
C) Ionization of Na requires no energy
D) Na and Cl are highly reactive due to small size

Answer: B (High lattice energy makes the process exothermic.)

Assertion Reason Type Questions With Answers and Explanation on Formation of Ionic Bond

Assertion (A): Alkali metals readily form ionic compounds.
Reason (R): Alkali metals have low ionization enthalpies.

A) Both A and R are true, and R is the correct explanation of A.
B) Both A and R are true, but R is not the correct explanation of A.
C) A is true, but R is false.
D) A is false, but R is true.

Answer: A (Low ionization enthalpy explains their high reactivity.)

Assertion (A): Lattice enthalpy of MgO is higher than that of NaCl.
Reason (R): The charge on Mg²⁺ and O²⁻ ions is higher than on Na⁺ and Cl⁻ ions.

A) Both A and R are true, and R is the correct explanation of A.
B) Both A and R are true, but R is not the correct explanation of A.
C) A is true, but R is false.
D) A is false, but R is true.

Answer: A (Greater ionic charges lead to stronger attraction and higher lattice enthalpy.)

Case Study based on Formation of Ionic Bond

Read the following passage and answer the questions:
The formation of NaCl from Na and Cl atoms involves three steps:
(i) Ionization of Na requires 495.8 kJ mol⁻¹.
(ii) Electron gain by Cl releases –348.7 kJ mol⁻¹.
(iii) Lattice formation releases –788 kJ mol⁻¹.
The net energy change is –640.9 kJ mol⁻¹, indicating an exothermic process.

Questions:

1. Which step is endothermic?
2. Why does NaCl form despite the high energy required for ionization?
3. What provides the major driving force for NaCl formation?

Answers:

1. Ionization of Na is endothermic.
2. Because the energy released during electron gain and lattice formation exceeds the energy absorbed in ionization.
3. High lattice enthalpy of NaCl provides the major driving force.