Crystal lattice models

Molecular model kits are designed to be re-used; models can be built and then taken apart again, but chemists often make permanent models of molecular structures for demonstrations or teaching. This crystallographic model of the metal beta-manganese was made by Mr C. E. Chapman, Chief technician of the Crystallography Department, part of the Cavendish Laboratory at the University of Cambridge, in about 1952. It shows how the atoms of manganese are arranged at high temperatures.

models-chem-betaman
Image 1 Model of the crystal lattice of beta-manganese; circa 1952. The red and green balls show atoms of manganese in different positions.Image © the Whipple Museum (Wh.5794).
unit cell
Image 2 Close up of the unit cell (the black wire frame) in beta-manganese.Image © the Whipple Museum (Wh.5794).

The structure of crystals

A crystal lattice is a repeating pattern of atoms in a solid. It is an ideal representation of how atoms are bonded together and where they are situated with respect to their neighbours within the solid.

This crystal lattice model (Image 1) shows the distances between atoms of manganese and their geometrical arrangement within the crystal structure. The black wire cube represents the smallest possible cube that describes the general arrangement of atoms in the crystal lattice. It is referred to in crystallography as the unit cell.

Beta-manganese

In many metals all of the atoms are the same distance apart and surrounded by 12 other atoms (much like the arrangement of stacks of oranges in supermarkets). In manganese however, for reasons not fully understood, this is not the case. At room temperature some of the distances are shorter than others. This makes the arrangement of managese atoms in the crystal lattice more complicated than most other metals.

When metallic manganese is heated the crystal lattice undergoes changes in its structure before the metal melts. These different states are known as allotropes G , and are named alpha, beta, gamma and delta to distinguish them. Between temperatures of about 700 to 1100 degrees centigrade, manganese exists in the beta state. As in the alpha state, not all the atoms are the same distance apart; but further, the atoms exist in two different geometrical arrangements (shown as red and green balls in Images 1 & 2). This does not mean that they are a different kind of atom, just that their neighbouring atoms are arranged differently.

Students of crystallography often find it difficult to understand the concepts of different geometrical sites in a crystal lattice, this problem is increased in a complicated crystal lattice such as beta-manganese. C. E. Chapman made this model especially to help a student visualise the arrangement of such sites. Models such as this are still used in teaching today.

James Hyslop

James Hyslop, 'Crystal lattice models', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, 2008 [http://www.hps.cam.ac.uk/whipple/explore/models/modellingchemistry/crystallatticemodels/, accessed 25 November 2017]

 
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