# Defects of Crystals

So while studying defects, I found that:

1. Interstitial defect is when constituent particle is present in the interstitial site. My question is where this constituent particle is coming from? As it leads to increase in density so It must come from outside?

2. In metal excess defect (due to anion vacancy) : In case of $\ce{NaCl}$ it says, when heated with sodium vapours, chloride ion leaves the crystal and combines with $\ce{Na}$ atom outside to release an electron. Why it is breaking the bonds with $\ce{Na}$ ion to again join with $\ce{Na}$ atom?