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-SulfurIn the structure to the left there are three sheets of magnesium atoms coloured grey. These sheets contain seven close-packed atoms arranged with six of them in a hexagonal shape around the central atom. The important thing to notice with these is that these alternating sheets have the atoms in the same positions on the plane, but just displaced in the perpendicular direction. Now consider the layers of green atoms between the grey sheets. These are also close-packed sheets that are in fact identical to the grey layers, though it is less obvious as only three atoms are shown in each of these layers. These green atoms are nestling in the gaps of the layer below. The important thing about these green layers is that the atoms they contain are also in equivalent positions, just displaced in the perpendicular direction. This is why the structure is described as AB: because there are only two relative positions of the close-packed layers within the structure (coloured grey and green on the left). This is different to cubic close-packing where there are three relative positions of close-packed layer (see page 4 of copper), and so the cubic close-packed structure is referred to as ABC in this notation.
The next page shows the unit cell in magnesium.