Sodium (body-centred cubic) 1: introduction

To go directly to the unit cell structure, click the link below to page 4.

Sodium is far too reactive to be found uncombined in nature. Indeed elemental sodium needs to be stored under oil to prevent it reacting with water and oxygen. Sodium chloride however is highly abundant, being the main salt responsible for the salinity of the oceans. Sodium is extracted from sodium chloride by electrolysis.

Sodium atoms are arranged in a cubic manner but they are not "close-packed" (see copper for what this means). Simple trigonometry shows that only 68% of space is occupied by hard spheres packed in a body-centred cubic arrangement, compared to 74% in close packing. That can't be appreciated looking at the structure to the left as the atoms are drawn with half their actual volume, so they are not touching. This is so that you can see inside the structure, which is impossible if the atoms are shown to be in contact. A space-filling representation is shown on the next page. Each atom in the bulk is in contact with eight neighbours, which are arranged at the eight corners of a cube around the original atom. The bonds in the structure on the left connect atoms that are touching in reality. Note that the number of nearest neighbours (the "coordination number") is less than it is for copper, which is twelve. The lower the coordination number the less efficient the packing in terms of the fraction of space that is filled. The structure to the left shows a segment of the bulk of sodium.

Go to page 2 to look at a space-filling representation of the structure of sodium.

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