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 blue wireframe just marks out the repeating unit of space in the structure - the lines are not supposed to depict any kind of chemical bond. Note that the unit cell is a prism of a rhombus, with angles of 60° and 120° in the rhombus.
Magnesium atoms in the bulk of the structure have a coordination number of 12, just as was the case with copper. This is not readily apparent from the unit cell as many connecting atoms are outside the cell. Note that only a fraction of each atom is actually inside the unit cell (with the exception of one atom that is fully inside) as the vertices of the cell are in the centres of the corner atoms. At a 60° corner the atom is shared between the 360°/60°=6 unit cells in the plane and the 6 unit cells in the adjacent plane; at a 120° corner the atom is shared between the 360°/120°=3 unit cells in the plane and the 3 unit cells in the adjacent plane. Therefore the atom occupancy in the unit cell is (4 x 1/12) + (4 x 1/6) + 1 = 2.
The next page shows the unit cell within the AB representation.