What is the difference between the number of atoms per unit cell in face-centred cube and the number of atoms per unit cell in body- centred cube?

Potassium crystallises in a $\mathrm{BCC}$ lattice. Coordination number of potassium will be:

What type of crystal defect is indicated in the diagram below?    

In a crystal the atoms are located at the position of ________.       

Suppose the mass of a single $\mathrm{Ag}$ atom is $‘\mathrm{m}’$. $\mathrm{Ag}$ metal crystallizes in $\mathrm{fcc}$ lattice with unit cell of length $‘\mathrm{a}’$. The density of $\mathrm{Ag}$ metal in terms of $‘\mathrm{a}’$and $\text{'}\mathrm{m}’$ is______.