In a simple cubic lattice,what percentage of the space is left unoccupied by the spheres?

What is the number of spheres in contact with a tetrahedral void?

In the face-centered unit cell,the lattice points are present at

In a $ccp$ structure of $X$ atoms,$Y$ atoms occupy half of the octahedral voids. If one $X$ atom and one $Y$ atom in each unit cell are replaced by a $Z$ atom,the molecular formula of the solid will be:

In crystalline silicon,$Si$ atoms occupy all the $ccp$ sites and every alternate tetrahedral void. The value of packing efficiency is closest to $....\%$

Atoms $A$ are arranged in an $FCC$ system,and atoms $B$ occupy all the octahedral voids and half of the tetrahedral voids. The formula of the compound is: