Schottky defect is observed in crystals when:

1. Some cations move from their lattice site to interstitial sites.
2. Equal number of cations and anions are missing from the lattice.
3. Some lattice sites are occupied by electrons.
4. Some impurities are present in the lattice.

HINT: The number of missing cations and anions are equal in Schottky defect.
Explanation:
 The Schottky defect is observed in the crystal when an equal number of cations and anions are missing from the lattice. Thus, the density of solid decreases.
When some cations move from their lattice site to the interstitial site is known as Frenkel defect. When some impurity is present on a crystal is known as an impurity defect.
When the lattice site is occupied by an electron, this type of defect is known as a metal excess defect. Hence, except (2) all statements are incorrect regarding the Schottky defect.