How do Atoms assemble into solid structure?

From structural point of view, there are three ways in which atoms can pack together. Accordingly, the materials are grouped as

• Crystalline,
• Amorphous and
• Semi-crystalline

In Crystalline structures, the atoms pack in a regular periodic form which is typical of metals, ceramics and for a very few polymers. A typical crystalline structure is shown below. Crystalline structures are often grouped into three categories: Single Crystal, Poly- Crystal and Liquid Crystals. Single crystal as the name suggests consists of only one large crystal of the material. Though it is very expensive to develop, single crystals being free from most of the defects show unusually high mechanical and other functional properties. Hence, in many specialised applications they are in very high demand. For example, Titanium based super alloys are mostly developed as single crystals to use in Turbine blades.

Crystalline Structure

Exactly opposite to the crystalline structures, the amorphous structure refers to completely irregular packing of atoms. Many polymers show this kind of packing. Also, rapid cooling of some of the materials from the liquid phase result in amorphous or glassy form of structure. A typical solid in amorphous form is shown below:

Amorphous Structure

Semi-crystalline structures are placed in between the two extremes. Some times glassy structures show local regularities. The degree of regularity determines the degree of crystallinity of such materials. Polymers like low density polyethylene (LDPE), Polystyrene etc. fall into this catagory.