Here are the answers to my last blog, 23rd July 2012,

Definitions of the words crystal and lattice:

A crystal is a solid material, with a regular three dimensional, repeating pattern to its structure. The dictionary definition adds that crystals have symmetrically arranged plane faces.

I usually say to my students crystals have flat surfaces and sharp edges. For example, sodium chloride forms a regular cubic structure in its crystals. They’re cubes!

Examples of lattices are a lattice fence and a lattice crusted pie. In both of these examples lattice refers to a criss-cross arrangement of strips (wood or pastry).

The word lattice applied to crystals means a regular, patterned structure. Again if you look it up in a dictionary it says a crystal lattice is “a geometric arrangement of the points in space”. In the case of sodium chloride the “points” are the sodium and the chloride ions.

There’s an old joke which goes “When is a door not a door?”

The answer is “When its ajar”.

Ajar means slightly open.

So “When is a crystal not a crystal?”


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