BAKER HUGHES - Drilling Fluids Reference Manual

Water Based Drilling Fluids
share. The electron orbitals are thus distorted towards them and the atoms develop partial
charges. Water is the best known example of this. In the water molecule, the oxygen atom
attracts far more than the two hydrogen atoms and the electrons are thus not shared equally. The
orbitals are distorted towards the oxygen which thus develops a partial negative charge, written
δ ⎯ . The hydrogen atoms develop a corresponding δ + charge. This type of covalent molecule is
said to be polar. Where the electrons are shared equally, no partial charges develop and the
molecule is non-polar. This is shown diagrammatically in
Figure 3-6
Polar and Non-Polar Covalent Bonding
The tendency to attract electrons within a covalent bond is known as electronegativity. The
electronegativity of an element depends on its position in the periodic table.
Fluorine is the most electronegative element. It is followed by oxygen, nitrogen, and chlorine, in
that order. The metals in groups IA and IIA are the least electronegative and are generally
referred to as electropositive elements due to their tendency to form positively charged ions. The
electronegative elements have the greatest tendency to form negative ions, though this is modified
by the number of electrons needed to be gained.
The larger atoms in groups IA and IIA lose their outer electrons very rapidly as they are a long
way from the nucleus and shielded from it by the other electron shells. Thus, cesium and barium
are more electropositive that sodium and magnesium respectively and more readily form cations.
The group IA metals are more electropositive than their group IIA counterparts, as only one
electron needs to be lost to form the stable ion. Thus potassium more readily forms K + ions, than
calcium does Ca 2+ ions.
The larger the difference between the electronegativity of two elements, the more likelihood there
is of ionic bonding between them. Alternatively, if the bond is covalent, the more polar it will be.
The electropositive elements in groups IA and IIA form strong ionic bonds and rarely, if ever,
form covalent bonds. The metals in the middle of the table (groups IVB – IIB) are weakly
electropositive and form either weak ionic bonds with the strong electronegative elements, or
polar covalent bonds with the weak electronegative elements. The weakly electronegative
elements form mainly covalent bonds but can form ionic bonds with the group IA metals. The
strongly electronegative metals form ionic bonds with electropositive metals or polar covalent
compounds with weakly electronegative elements.
Reference Manual
Baker Hughes Drilling Fluids
3-10 Revised 2006