Question

Consider the following assumptions made while developing the ionic packing theory:

1. Cations and anions are spherical but these spheres are not hard

2. Cations are always smaller than anions

3. Each cation would tend to be surrounded by the maximum number of anions permitted by the geometry

4. Cations and anions do not touch each other 

Which of the above assumptions are correct?

1. 1 and 3
2. 1 and 4
3. 2 and 4
4. 2 and 3

Answer 1

Correct Answer - Option 4 : 2 and 3

• In an ionic solid, the ions are packed together into a repeating array called a crystal lattice.
• The concept of crystal packing assumes that the ions are hard spheres. (Statement 1 is incorrect)
• The easiest way to picture such an array is to arrange one layer of spheres and then place successive layers over it.
• In most ionic compounds, the anions are much larger than the cations, and it is the anions which form the crystal array. The smaller cations reside in the holes between the anions. (Statement 2 is correct)
• Ions try to surround themselves with as many ions of opposite charge as closely as possible. Usually, in the packing arrangement, the cation is just large enough to allow the anions to surround it without touching one another. (Statement 3 is correct)
• In ionic crystals, ions of opposite charge touch each other but ions of the same charge should not touch. (Statement 4 is incorrect)