Carbon, silicon, germanium and tin are atoms in ascending order of atomic number from column IV A of the period table. Each is characterised by having four valence electrons in its outermost shell of electrons, and requires a further four to make up the full complement of the shell. All can solidify to form elemental, covalently bonded crystals where the four valence electrons of one atom are shared between its four nearest neighbours so that every atom effectively gains eight electrons in its valence shell. A group IV atom and its four nearest neighbours from a tetrahedron as shown in Figure 1.

Figure 1: Schematic diagram to show the orientation of covalently bonded group 4 atoms. A tetrahedron is formed by the nearest neighbours, with the principal atom located at its centre.

Figure 2: Unit cell of a crystal such as silicon or germanium.

Taking a larger scale perspective of the arrangement of the atoms, or crystal lattice, it is found that they organise themselves into two interpenetrating face centred cubic (fcc) sub-lattices, one displaced from the other by 1/4(a0, a0, a0) along a diagonal of the unit cell. a0 is called the lattice constant or lattice parameter and is a measure of the size of the unit cell, often expressed in Ångstrom (Å) units (1Å=0.1nm or 1x10-10m). It is determined by techniques such as X-ray diffractometry. Figure 2 shows a complete unit cell for a group 4 crystal covalently bonded with the diamond structure.

This structure is of course difficult to visualise and draw, hence it is usually represented by an equivalent 2-D, "square" arrangement shown in figure 3.

Figure 3: 2-D representation of a covalently bonded crystal at 0K, eg Si. Note that the heavy lines between adjacent atoms depict the covalent bonds which contain TWO electrons and are all completely filled.

In the example shown, it is assumed that the solid, Si say, is both crystallographically perfect and pure. At 0 K, all the covalent bonds are complete and there are no free charge carriers moving around randomly through the lattice; the crystal is an insulator.