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-SulfurOther compounds that adopt this structure: SrF2, BaF2, PbF2, CdF2, HgF2, ZrO2, HfO2, ThO2, CeO2, PrO2 and 3d metal dihydrides.
The unit cell of fluorite is shown to the left. The black wireframe just marks out the repeating unit of space in the structure - the lines are not supposed to depict any kind of chemical bond. The fluoride ions are shown in yellow and the calcium ions in grey. The calcium ions adopt a face-centred cubic arrangement, ie the ions occupy the eight corners of the cubic unit cell and the centres of the six faces (analogous to cubic close-packed). There are eight fluoride ions lying inside the cube, coordinated to (ie in contact with) four neighbouring calcium ions. These four ions lie in a tetrahedral arrangement around each fluoride ion. So altogether there are 22 ions involved with the unit cell, but the cell occupancy is less than this since many of the ions are only partially inside the cell.
Cell occupancy:
The cell occupancy of calcium ions = (8 x 1/8) + (6 x 1/2) = 4.
The cell occupancy of fluoride ions = 8.
The cell occupancy reflects the 1:2 stoichiometry of calcium and fluorine in the formula of calcium fluoride.
Coordination numbers:
The coordination number of fluoride is 4, ie it has four nearest neighbours. The coordination number of calcium is 8. This is less easy to observe in the unit cell but if you consider that the calcium ions in the corner positions are shared between eight unit cells, it makes sense as the calcium is in contact with one neighbouring fluoride in each cell. The face-centre calcium ions are in contact with four neighbouring fluoride ions in the unit cell and another four in the adjacent cell (face-centre ions are shared between two unit cells). Note that the nearest neighbours of an ion are the counter ions of opposite change. This is how the structure maximises the attractive ionic forces between ions of opposite charge.
Go to page 4 to consider the stacking of unit cells.