# The density of CaO is $3.35 gm/cm^3$. The oxide crystallises in one of the cubic systems with an edge of 4.80 $A^{\large\circ}$. How many $Ca^{++ }$ ions and $O^{–2}$ ions belong to each unit cell, and which type of cubic system is present?

No of $Ca^{++}$ion=4,No of $O^{--}$ion=4,fcc