Given below are two statements.
Statement $I:$ Frenkel defects are vacancy as well as interstitial defects.
Statement $II:$ Frenkel defect leads to colour in ionic solids due to presence of $F$-centres.
Choose the most appropriate answer for the statements from the options given below:

Explain Schottky defect and Frenkel defect.

In an ionic crystal,$F$-centers are.....

Explain the following term with a suitable example:
$(iii)$ Interstitials

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R).$
Assertion $(A):$ In a particular point defect,an ionic solid is electrically neutral,even if few of its cations are missing from its unit cells.
Reason $(R):$ In an ionic solid,Frenkel defect arises due to dislocation of cation from its lattice site to interstitial site,maintaining overall electrical neutrality.
In the light of the above statements,choose the most appropriate answer from the options given below:

Explain the metal excess defect.