Inspite of long range order in the arrangement of particles why are the crystals usually not perfect ?

In spite of long range order in the arrangement of particles why are the crystals ususally not perfect?

A crystal has an ordered arrangement of constituent particles: Define the term unit cell.

A crystal has an ordered arrangement of constituent particles:Copper crystal has a' face centred cubic lattice structure. Its density Is 8.93gcm^-3 . What is the length of the unit cell ? Atomic mass of copper is 63.5 gmol^-1 .

A crystal has an ordered arrangement of constituent particles: Calculate the number of atoms in simple cubic and body centred unit cells.

The sharp melting point of crystalline solids compared to amorphous solids is due to a) a regular arrangement of constituent particles observed over a short distance in the crystal lattice. b) a regular arrangement of constituent particles observed over a long distance in the crystal lattice. c) Same arrangement of constituent particles in different directions. d) Different arrangement of constituent particles in different directions.

Which type of solids have long range order and sharp melting point ?