Magnesium (hexagonal close-packed) 1: close-packing

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

Magnesium is too reactive to be found uncombined in nature. It is however the eighth most abundant element in the earth's crust and the third most abundant element dissolved in the oceans. It is commonly extracted from its chloride using electrolysis. The element is notable for the brilliant white light it emits when it burns, which is why it is used in fireworks and flares.

Magnesium atoms are "close-packed" in a related but not identical way to how copper atoms are packed in cubic close-packing. Hexagonal and cubic close-packing are equally efficient methods of packing hard spheres into a lattice, both occupying 74% of the available space. The structure to the left doesn't show hard spheres in contact, even though that approximates what it actually is. Instead the atoms are drawn with half their actual volume. That is because you can't see inside the bulk with a realistic space-filling representation, which is shown on the next page. Also, showing the structure as atoms connected by bonds makes it easier to work out structural characteristics, as bonds are only drawn between atoms that can be considered to be touching. With close-packing, the coordination number for an atom in the bulk is maximised. The coordination number of an atom is the number of atoms in contact with it. Look at the bulk structure of magnesium on the left and try to see how many nearest, ie touching, neighbours a magnesium atom in the bulk has.

Go to page 2 to look at the space-filling representation of magnesium's structure.

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