Short Answer

Smaller ions have higher lattice energy because they can come closer to each other in an ionic crystal. When ions are closer, the electrostatic attraction between the positive and negative ions becomes much stronger. This strong attraction releases more energy when the lattice forms, increasing lattice energy.

As ionic size decreases, the distance between ions decreases, so the force of attraction increases. This is why compounds with small ions, such as LiF, have higher lattice energies compared to compounds with larger ions like CsF.

Detailed Explanation :

Why Smaller Ions Have Higher Lattice Energy

The lattice energy of an ionic compound depends on how strongly its cations and anions attract each other in the solid lattice. According to Coulomb’s law, the strength of attractions between ions is inversely proportional to the distance between them. This means the closer the ions are, the stronger the attraction and the greater the lattice energy will be.

Smaller ions have smaller ionic radii, allowing them to pack tightly in the crystal structure. This reduces the distance between the centers of opposite charges. When ions are closer, the electrostatic forces become stronger, making the lattice more stable and increasing the energy released during lattice formation. This principle explains why ionic compounds with small ions tend to have high melting points, high hardness, and strong ionic bonds.

1. Relationship Between Ionic Size and Electrostatic Attraction

The electrostatic attraction between ions is explained by Coulomb’s law:

Where:

•  and  are ionic charges
•  is the distance between ions

For smaller ions:

•  is small
• Force of attraction becomes very strong

This strong attraction means more energy is released when ions come together, which increases lattice energy.

2. How Smaller Ions Pack in the Crystal Lattice

Smaller ions:

• Fit closer together
• Form a more compact lattice
• Allow more efficient packing

Since the ions are tightly held, breaking the lattice into gaseous ions requires a large amount of energy.
Thus, compounds with smaller ions are more stable and resistant to melting and dissolving.

Example:

• LiF has high lattice energy because Li⁺ and F⁻ are both small.
• CsF has lower lattice energy because Cs⁺ is much larger.

3. Effect of Ionic Size on Lattice Stability

Smaller ions create a more stable lattice because:

• Shorter distance → stronger attraction
• Greater stability → higher lattice energy

This is why:

• MgO (small Mg²⁺ and O²⁻ ions) has an extremely high lattice energy
• NaCl (larger Na⁺ and Cl⁻ ions) has moderate lattice energy

Thus, ionic size is a major factor in determining the stability and strength of ionic solids.

4. Trends in the Periodic Table

Across a Period

Ions become smaller as you move left to right in a period due to increasing nuclear charge.
Smaller ions → higher lattice energy.

Example:
LiF > NaF > KF > RbF > CsF

Down a Group

Ionic size increases down a group.
Larger ions → lower lattice energy.

Example:
F⁻ < Cl⁻ < Br⁻ < I⁻ (size increases → lattice energy decreases)

5. Connection to Melting Point and Physical Properties

Higher lattice energy caused by smaller ions leads to:

• Higher melting point
• Greater hardness
• Reduced solubility in water
• Increased thermal stability

For example:

• MgO, with small ions, melts at over 2800°C.
• NaCl, with larger ions compared to MgO, melts at around 801°C.

This shows how ionic size influences lattice energy and, therefore, physical behavior.

6. Why Smaller Ions Form Stronger Ionic Bonds

Smaller ions:

• Have high charge density
• Attract opposite ions more strongly
• Create stronger ionic bonds

High charge density means the charge is concentrated in a small volume, intensifying attraction.
This strengthens the ionic bond and increases lattice energy.

Example:
Be²⁺ is much smaller and more strongly polarizing than Ca²⁺.
Thus, compounds of Be²⁺ tend to have higher lattice energy and more covalent character.

7. Examples That Demonstrate the Effect of Ionic Size

• LiF has a high lattice energy because both ions are small.
• KI has lower lattice energy because I⁻ is very large.
• MgO has extremely high lattice energy because Mg²⁺ and O²⁻ are both small and highly charged.

Each example shows how reducing ionic size increases the strength of attraction.

Conclusion

Smaller ions have higher lattice energy because they allow shorter distances between oppositely charged ions, increasing electrostatic attraction. This stronger attraction makes the ionic lattice more stable and requires more energy to separate the ions. As a result, compounds with smaller ions have higher melting points, greater hardness, and stronger ionic bonds. The relationship between ionic size and lattice energy explains many important physical and chemical trends in ionic compounds.