BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Atoms which gain electrons, and thus become negatively charged ions, are called anions. Thus
Cl ⎯ is the chloride ion (one negative charge) and S 2⎯ is the sulfide ion (two negative charges, i.e.,
two electrons gained).
Groups of atoms can also gain or lose electrons to become ions. For example, SO 2⎯ 4 is the sulfate
ion. CO 2⎯ 3 is the carbonate ion and NH + 4 is the ammonium ion. A full list of these types of ions
is given in Table 3-3.
When an ionic bond forms, electrons are transferred in such a way that each atom attains a
complete outer shell of electrons. When one electron is lost, the remaining electrons are held
more tightly by the nucleus and it becomes progressively more difficult to remove them. True
ionic bonds rarely exist when the transfer involves 3 or more electrons, as this procedure requires
too much energy.
Thus sodium, with one electron in its outer shell readily loses this electron to form the Na + ion.
Chlorine, with seven electrons in its shell, readily accepts an electron to form the Cl ⎯⎯ ion. The
compound sodium with chlorine, sodium chloride, is ionic and can be written Na + Cl⎯, in which
each ion would again have a full outer electron shell. The formation of sodium chloride is
illustrated schematically below.
Figure 3-2
Covalent Bonding
The Formation of Sodium Chloride as an Example of Ionic Bonding
Where electrons are shared between atoms, the bond said is to be covalent. This is illustrated in
below for water and carbon dioxide. By this sharing process, the electron clouds of the atoms
overlap such that each atom thinks it has a full outer electron shell.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-7