Page 132 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Minerals
Soil Minerals 127

Covalent bonds are directional, and generally are stronger than ionic bonds.
Diamond and quartz are heavily imbued with covalent bonds, and these minerals
are hard and obviously do not dissolve in water. Unlike ionic attractions, covalent
bonds are highly directional as they involve orbiting electron shells.

Many minerals incorporate both ionic and covalent bonding. Calcium carbonate,
2þ
for example, is composed of calcium ions, Ca , which are held ionically to
2þ
2
2
covalently bonded carbonate, (CO 3 ) , to make Ca (CO 3 ) , or CaCO 3 . The
ionic bond is easily attacked by a weak acid so calcium carbonate dissolves, but
the covalently bonded CO 2 stays together and escapes as gas.
6.3.2 Part-Time Ions
Atomic pairs that are most likely to lose or gain electrons are those at the opposite
edges of the periodic table. Na and Cl, for example, are at the left and right edges
respectively. Bonds between closer atom pairs are less ionic and more covalent, as
in the case of Si and O, which make up SiO 2 , quartz. Covalent bond percentages
were quantified by Linus Pauling, who estimated the Si—O bond to be
approximately 50 percent ionic and 50 percent covalent.

6.3.3 Some Unique Aspects of Water

Water is a unique molecule in that a hydrogen atom that loses an electron
becomes a proton, a tiny, intense positive charge. A proton exposed on the surface
of the oxygen atom in a water molecule is highly attractive to the oxygen atom of
another water molecule. Even though the two molecules are electrically neutral,
this can build a loose structure as a result of ‘‘hydrogen bonding.’’

Hydrogen bonding causes an open lattice structure when water freezes to ice.
When ice melts this structure collapses, which is a unique property of water. Were
it not for this unique property, polar ice would sink and accumulate at the bottom
of the sea. Also, applying pressure to ice to increase the density can cause it to
melt, which is important for moving glaciers, sleds, and ice skates.

Another factor affecting water is the angle between two hydrogen atoms, which is
fixed by the orbital geometry at about 1058. Each water molecule therefore has a
positive end and a negative end. This dipolar nature causes water to be attracted
þ
to other ions, particularly small, positively charged ions such as Na .
Na þ and Ca 2þ ions are associated with clay minerals and carry with
them adsorbed hulls of water that can influence the expansive properties.
These ions also affect the net electrical charge on clay particles and affect
whether they are attracted to one another, or flocculated, or repel and are
dispersed. These properties can dramatically influence the engineering properties
of a clay.

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