7) in a NiO cubic crystal unit cell there are 4Ni and 40 atoms. The atomic weights and atomic radii of Ni and O are: 58.71g and 16g, and 0,69A^ and 1,4A^, respectively. a) Find the specific gravity of the NiO compound. b) if some of the Ni atoms are missing in the NiO compound, the specific gravity decreases by 9%. Calculate what percentage (\%) of the unit celis have a Ni atom missing for such a reduction in specific gravity? (Avogradro's Number 61023, l2 104cm, Hint: You can take the crystal structure of NiO to be the same as NaCl,MgO or BaO )