Explain vacancy defect and interstitial defect.

(N/A) $(i)$ Vacancy defect: When some of the lattice sites are vacant,the crystal is said to have a vacancy defect.
- This results in a decrease in the density of the substance.
- This defect can also develop when a substance is heated.
- $(ii)$ Interstitial defect: When some constituent particles (atoms or molecules) occupy an interstitial site,the crystal is said to have an interstitial defect.
- This defect increases the density of the crystal.
- Vacancy and interstitial defects are found in non-ionic solids.
- In ionic solids,electrical neutrality must be maintained; therefore,instead of vacancy and interstitial defects,these are known as Frenkel and Schottky defects.