Amorphous solids have no long-range order. The atoms or molecules in these solids are not periodically located over large distances. An amorphous structure is shown below.
Many amorphous materials have internal structures similar to liquids. In fact, the only obvious distinction between amorphous materials, such as glass, and liquids is the high viscosity (resistance to flow) of the amorphous solids.
All solids tend to exist in the crystalline state rather than the amorphous state because the crystalline structure always has a larger binding energy. However, in numerous instances amorphous solids are formed when liquids are cooled below the melting temperature. This occurs for two reasons:
1) the structure of the molecules is so complex that they cannot easily rearrange themselves to form a crystalline structure, and/or
2) the solid forms so rapidly that the atoms or molecules do not have time enough to rearrange themselves in a crystalline structure.
Generally, amorphous solids have one of two distinct atomic arrangements:
either a tangled mass of long-chained molecules or a 3-dimentional network of atoms with no long-range order.
Amorphous materials with long-chained molecules (e.g. polymers) have a structure similar to that shown below.
Each segment in above figure represents one of the repeating units of the polymer chain.
The arrangement of the molecules is fairly random, resulting in a loosely packed structure. Network amorphous solids are usually Oxides, the most common being Silica (SiO2). The amorphous SiO2 structure is shown below.
Oxygen atoms are shown (corners of tetrahedral) in this amorphous SiO2
structure. There is a Silicon atom at the center of each tetrahedral which is
not shown here.
This structure has short-range order but none of the long-range order found in crystalline Silica. Thus, in both amorphous and crystalline Silica, each Silicon atom and each Oxygen atom have essentially the same local surroundings, even though there is no long-range periodicity in the amorphous structure.