CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
The mineral sodium fluoride, NaF, is an ionic solid in which each ion is surrounded by six counter-ions (the same rock-salt motif found in NaCl). The ionic radii are 102 pm for Na⁺ and 133 pm for F⁻.
Assume that the ions are rigid spheres and that electrostatic interactions dominate the lattice energy.
(a) Predict the most probable coordination number for Na⁺ in NaF if the radius of F⁻ were hypothetically increased to 175 pm while the radius of Na⁺ remained unchanged. Support your prediction by referencing the concept of radius ratio.
(b) On a blank sheet of paper, sketch a single close-packed layer (a two-dimensional slice) of the NaF lattice as it actually exists. Represent cations and anions with circles whose relative sizes reflect their ionic radii, and indicate at least one complete Na⁺F⁻Na⁺ edge in your drawing.
(c) Explain how the arrangement you sketched in part (b) is consistent with Coulomb’s law in terms of maximising attractive interactions and minimising repulsive interactions among the ions present.
(d) If the F⁻ radius were decreased by 15 % from its actual value (133 pm) while the Na⁺ radius stayed the same, discuss whether the rock-salt arrangement would still be favoured. Justify your reasoning qualitatively without numerical calculations.