Bonding and Melting Point
Name: Scott B.
Date: Friday, April 26, 2002
While observing a chemistry class on chemical bonding the
teacher ended the lesson with the statement "Ionic bonds are generally
stronger than covalent bonds that is why ionic materials melt at higher
The statement has two parts,
First "Ionic bonds are stronger generally than covalent bonds"
Second "that is why ionic materials melt at higher temperatures"
What is the truth here?
Can any such generalizations be made about Ionic and Covalent bonds and
their melting points?
I can think of many contradictory examples. I have several texts on
mineralogy that specifically say that covalent bonds are stronger than
ionic because they are directional and most of the bond energy in
concentrated in the area of the shared electrons where as ionic bonds are
a general attraction of + and - charged atoms due to the almost complete
transfer of the electron. Is this true for all materials like say CO2 or
O2 or just covalently bonded solids?
Bond dissociation energies (the energy required to separate the bonded atoms
to an infinite distance apart) for ionic bonds are generally higher for ionic
compounds than for covalent compounds. In fact, as a good general, rule,
the more ionic character a covalent bond has, the stronger it is. This
observation by Linus Pauling led to his electronegativity scale.
However, this says nothing about melting points. Ionic solids are arranged
into lattices in which each charged species is surrounded by species of
opposite charge. To disrupt such a lattice generally means introducing
unfavorable like-like charge interactions and disrupting favorable opposite
charge interactions. When covalent solids melt, usually the covalent bonds
themselves can stay intact. For instance, when water melts, the water
molecule H-O-H keeps its connectivity and shape. It's the weaker,
non-covalent interactions between different water molecules that are
disrupted when diamond melts.
Some covalent solids, however, consist of essentially enormous covalent
molecules. Such compounds include quartz, diamond, and silicon carbide.
These solids cannot be divided into discrete molecular units. The chains of
covalent bonds continues in all directions without end. To melt such
substances requires quite high temperatures.
To further illustrate ho you cannot make ionic/covalent generalizations about
melting points, I need to mention that there is a very large class of
compounds known as room temperature ionic liquids, or room-temperature
molten salts. These consist of compounds in which the anion or cation
(sometimes both) are molecules in which the charge is spread out over
several atoms. The cation and anion do not readily form a neat crystalline
lattice, so they are actually liquid down to room temperature or below.
Now, these salts do no vaporize at accessible temperatures (unless they
decompose), but they most definitely do not have high melting points.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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