BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Properties of Compounds
Ionic Compounds
Ionic compounds do not generally form discrete molecules. Instead, the ions tend to arrange
themselves in a three dimensional lattice structure with each other surrounded by anions and vice
versa. The relative number of cations to anions is the same as is written in the chemical formula.
You will find the structure of sodium chloride illustrated in Figure 3-7.
Figure 3-7
The Sodium Chloride Structure
The ions are held together in the lattice by strong electrostatic forces (ionic bonds) and it is
difficult to force them apart. Thus ionic compounds are usually solids with high melting points
and boiling points. The energy required to separate the ions is called the lattice energy. Lattice
energies are usually quite large compared to other bond energies.
Covalent Compounds
In covalent compounds the bonds that hold together are usually quite strong, but there are strong
forces holding the molecules together. These low intermolecular forces mean that covalent
compounds tend to be liquids or gases, as the molecules are not sufficiently attracted to each
other to hold them in place in a solid structure against the random molecular motion produced by
thermal energy.
The heaver the molecules, the more thermal energy is usually needed to separate them. Thus,
compounds with high molecular weight usually have higher melting points and boiling points.
The degree of polarity has a very important influence on intermolecular forces. In polar
compounds these are increased by the attractions between the opposite partial charges, as shown
in Figure 3-8.
The polar attraction between molecules explains why water is a liquid, although having a lower
molecular weight than Hydrogen Sulfide, which is a gas. (see Table 3-5)
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-11