MCQ Detailed View

Lattice energy is the energy released when gaseous ions come together to form an ionic solid, or alternatively, the energy required to separate one mole of an ionic solid into its gaseous ions. It is a direct measure of the strength of ionic bonding in a crystal lattice.

Two key factors influence lattice energy:

1. 
   

   Charge of ions – Higher ionic charges result in stronger electrostatic attraction, leading to higher lattice energy.

   
   


2. 
   

   Ionic radii (size of ions) – Smaller ions can pack more closely together, increasing attraction and lattice energy. Conversely, larger ions are farther apart, resulting in lower lattice energy.

   
   

In the given options:

• 
  

  NaF involves small ions (Na⁺ and F⁻), so it has very high lattice energy.

  
  


• 
  

  NaCl has a slightly lower lattice energy than NaF, since Cl⁻ is larger than F⁻.

  
  


• 
  

  KCl has still lower lattice energy because K⁺ is larger than Na⁺, reducing attraction.

  
  


• 
  

  CsI involves the largest cation (Cs⁺) and one of the largest anions (I⁻). Their large sizes mean weaker electrostatic attraction and, therefore, the lowest lattice energy among the choices.

  
  

This trend can be summarized by the Born–Lande equation, which shows lattice energy is inversely proportional to the sum of ionic radii and directly proportional to the product of ionic charges. Since all the compounds listed involve monovalent ions (charge ±1), size dominates.

In practical terms, compounds with low lattice energy (like CsI) are more soluble in water and have lower melting points compared to those with high lattice energy (like NaF). This makes lattice energy an important concept in predicting physical properties such as solubility, hardness, and stability of ionic compounds.

Thus, CsI has the lowest lattice energy among the given compounds.