STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

298 General Engineering and Science
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2Hx + -6: + H":
H
X.
(water, H,O)
where the symbols x and - represent electrons in the outermost shells of the atoms.
Covalent compounds are mostly organic. In compounds containing both ionic and
covalent bonds (e.g., KNO,, MgSO,, etc.), the physical properties are primarily
determined by the ionic character.
Polarity in Covalent Compounds
A covalent bond will exhibit polarity when it is formed from atoms that differ in
electronegativity, i.e., the ability to attract electrons. The order of electronegativity
of some elements [50, p. 161 is
F> 0 > N > C1> Br > C > S > I > H > B > Si, Zn > Mg> Li, Na, Ky Sn
As an illustration, in the iodine monochloride (I - Cl) molecule, the electron pair
being shared remains closer to the more electronegative C1 atom. This creates
fractional negative and positive charges, referred to as formal charges, on the C1 and
I atoms, respectively, as illustrated below:
*I CIS-
The symbols 6+ and 6- indicate polarity of the two ends or poles of the electrically
neutral molecule. Such a polar molecule constitutes a permanent dipole,
i.e., two equal and opposite charges (e) separated by a distance (d) in space. A
quantitative measure of the polarity of a molecule is the dipole moment (p in
Debye units), which is defined as the product of the charge (e in electrostatic
units) and the distance (d in cm).
Polarity of bonds can lead to polarity of molecules, as shown in the case of the
water molecule:
/O\
*H
H"
(bond angle = 105")
Nonpolar molecules such as H,, N,, O,, I,, and C1, have zero dipole moments, because
e = 0. On the other hand, hydrogen fluoride, HF, has a large dipole moment of 1.75
Debye and so is strongly polar. Simple carbon compounds with symmetric arrangement
of like atoms (e-g., methane, CH,, and carbon tetrachloride,CCl,) have zero dipole
moments and so are nonpolar.
In polar compounds, the operative intermolecular forces are dipole-dipole
interactions, which refer to the attraction between the positive pole of one molecule
and the negative pole of another. For this reason, polar compounds are relatively
more stable than nonpolar substances. A particularly strong kind of dipole-dipole
attraction is hydrogen bonding, in which a hydrogen atom acts as a bridge between
two electronegative atoms, holding one atom by a covalent linkage and the other by
purely electrostatic forces, e.g.,