Giant Metallic

This section deals with the arrangements of atoms in metals, the nature of metallic bonding and the properties of metals. All the atoms of a sample of a pure metal are identical and we can regard them as spheres.

Close Packing of Atoms
of equal size
- view from the top -

Click through the sequence below:

Add a Second Layer of Atoms
Add a Third Layer of Atoms
(first possibility)

Add a Third Layer of Atoms
(second possibility)

The diagram above illustrates a single layer of close-packed atoms placed on a gree surface. Consider the atom marked with an x. Each atom is surrounded by six others in the same layer. If there were layers of atoms placed above and below this layer, then each atom would be "touching" a total of 12 other atoms. x is said to have 12 coordination. Atoms that "touch" must be bonded to each other if they are to remain in "contact".

The diagrams above use different colours for the atoms of the different layers and they are viewed from the top. The same close packing structures are illustrated below, but this time different colours are used for atoms within a layer in order to emphasise the cubic close packing arrangement. The view is more from the side.

Hexagonal Close Packing Cubic Close Packing
Pack Layers
Pack Layers
Rotated Unit Cell
The diagram above represents an 'exploded' view of the hexagonal close-packed arrangement. The diagram above represents an 'exploded' view of the cubic close-packed arrangement.

Not all metals have a close-packing arrangement of their atoms. A third possible arrangement of atoms is body-centred cubic (b.c.c.). Here the structure is basically cubic with an atom at the centre of each cube. More space is wasted in this more open structure as the co-ordination number of each atom is eight. Na, K, Ba, U and Mn have the b.c.c. structure. There seems to be no clear relationship between the crystal structure of a metal and its position in the Periodic Table.

The diagram below shows a layer of atoms in a body-centred cubic arrangement. Layers can be added to build up the b.c.c. structure. The different coloured spheres again are to emphasise the b.c.c. arrangement.

Body-Centred Cubic Packing
Add a Second Layer
Add a Third Layer
Add a Fourth Layer
Add a Fifth Layer

The Unit Cell of a body-centred cubic structure is normally represented as shown below:

However, each atom at a corner of the cube is shared by eight unit cells and so contributes only 1/8 to a particular unit cell. The atom at the centre of a unit cell belongs entirely to that unit cell. Therefore, the total number of atoms that make up a unit cell is 2.


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