Calcium fluoride, CaF2, crystallizes so that each Ca2+ ion is coordinated by eight F− ions. Which of the following best accounts for the stability of this unusually high coordination number?
a) The small ionic radius of F− allows eight of them to approach a Ca2+ closely without significant anion–anion repulsion.
b) The rF−rCa2+ radius ratio exceeds the 0.732 threshold that permits a cubic site with eight nearest neighbours.
c) The small ionic radius of Ca2+ lets it fit comfortably inside a cube of eight fluoride ions, minimising repulsions.
d) Each Ca2+ follows the octet rule by surrounding itself with eight F− ions, which maximises lattice stability.