Q.In calcium fluoride, having the flurite structure the coordination number of $Ca^{2+}$ ion and $F^-$ Ion are (NEET)

Building close-packed layers in the ABAB... pattern (Section 6.6.2, each sphere touching 6 neighbours) and stacking a third layer over them in two different ways gives the two most efficient 3D packings possible:

• aba stacking -> hexagonal close-packed (hcp): the third layer sits directly over the first.
• abc stacking -> cubic close-packed (ccp): the third layer is offset from BOTH earlier layers (built on the fcc unit cell).

Both reach coordination number 12 (6 in-layer + 3 above + 3 below) and a packing efficiency of ~74% -- the highest of any arrangement in this unit (derived from the fcc relation 4r = root-2*a, giving r = (root-2/4)a). …