Calcium fluoride, CaF

CHEMISTRY · Single correct · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties

Problem

_2

, crystallizes so that each Ca

^{2+}

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

\mathrm{F^-}

allows eight of them to approach a

\mathrm{Ca^{2+}}

closely without significant anion–anion repulsion. (b) The

\dfrac{r_{\mathrm{Ca^{2+}}}}{r_{\mathrm{F^-}}}

radius ratio exceeds the

0.732

threshold that permits a cubic site with eight nearest neighbours. (c) The small ionic radius of

\mathrm{Ca^{2+}}

lets it fit comfortably inside a cube of eight fluoride ions, minimising repulsions. (d) Each

\mathrm{Ca^{2+}}

follows the octet rule by surrounding itself with eight

\mathrm{F^-}

ions, which maximises lattice stability.