Animated Solution for Chemistry - States of Matter: What type of crystal defect is indicated in the diagram shown below?

Reality

Imagine you are shrinking down to the atomic level and walking through a crystal of table salt, Sodium Chloride ($NaCl$). In a perfect, ideal world, you would see a beautiful, endless, alternating pattern of positive sodium ions ($N{a}^{+}$) and negative chloride ions ($C{l}^{-}$). It's a perfectly balanced architectural marvel.

However, nature is rarely perfect. When crystals form, especially at high temperatures, mistakes happen. These "mistakes" or imperfections in the regular arrangement of constituent particles are what we call crystal defects.

Let's look closely at the diagram provided in the problem. If you trace the rows and columns, you'll notice that the alternating pattern is broken.

In the second row, there is an empty spot where a positive sodium ion ($N{a}^{+}$) should be.

Similarly, in the third row, there is another empty spot, but this time, it's a negative chloride ion ($C{l}^{-}$) that is missing.

We have exactly one missing cation and one missing anion.

This specific scenario—where an equal number of cations and anions are completely missing from their regular lattice sites—is the textbook definition of a Schottky defect.

Why must they go missing in pairs? It all comes down to electrical neutrality. A crystal must always remain electrically neutral. If only a positive sodium ion went missing, the entire crystal would suddenly have a net negative charge, making it highly unstable. By losing one positive charge and one negative charge simultaneously, the crystal maintains its delicate electrical balance.

Schottky defects don't just happen in any random crystal. They are typically observed in highly ionic compounds that meet two specific criteria:

1. High Coordination Number: The ions are surrounded by many oppositely charged neighbors. 2. Similar Ionic Sizes: The cations and anions are roughly similar in size (e.g., $NaCl$, $KCl$, $CsCl$).

Because actual mass is lost from the crystal (the ions are gone!) while the overall volume of the lattice remains roughly the same, a Schottky defect inherently decreases the density of the solid.

In contrast, a Frenkel defect occurs when an ion simply dislocates from its normal site and hides in an interstitial space (a gap between other ions). Since no mass is lost, the density remains unchanged.

Given our visual evidence of missing pairs, the answer is definitively a Schottky defect.