Resources written by Chas McCaw for sixth form chemistry teaching and beyond.
General interest:
Graphite Buckminsterfullerene Ice White phosphorus Benzene Cyclohexane AdamantaneCubic:
Sodium Caesium chloride Polonium Copper Halite Fluorite Antifluorite Zinc blende DiamondNon-cubic:
Hexagonal:
Magnesium WurtziteTetragonal:
RutileTrigonal:
α-quartzTriclinic:
Copper(II) sulfateOrthorhombic:
α-SulfurMonoclinic:
β-SulfurThe image from the previous page is reproduced with only the critical atoms showing; the fluoride ion is virtually in contact with its neighbours. You will need to rotate the tetrahedron so that a triangular face of calcium ions is oriented horizontally, with the fluoride ion between it and the remaining calcium ion above. This shows how the central fluoride ion nestles between the calcium ions from close-packed layers above and below.
In a way it's slightly perverse to think of the fluoride ions occupying holes in a close-packed lattice of calcium ions since the fluoride ions are larger than the calcium ions, and so the supposedly close-packed array of calcium ions is actually opened up to quite a large extent by the large anions occupying its holes. So why is this representation chosen? It's because we are making the best use of the available symmetry by considering the structure to be based around cubic close-packing.
The following pages take a more advanced approach.
Go to page 7 to consider a polyhedral representation of the lattice.