Melting point
In a crystalline solid the particles acting as structural units-ions or molecules-
are arranged in some very regular, symmetrical way; there is a geometric
pattern repeated over and over within a crystal.
Melting is the change from the highly ordered arrangement of particles in the
crystalline lattice to the more random arrangement that characterizes a liquid (see
Figs. 1.1 8 and 1.19). Melting occurs when a temperature is reached at which the
thermal energy bf the particles is great enough to overcome the intracrystalline
forces that hold them in position.
An ionic compound forms crystals in which the structural units are ions. Solid
sodium chloride, for example, is made up of positive sodium ions and negative
chloride ions alternating in a very regular way. Surrounding each positive ion andequidistant from it are six negative ions: one on each side of it, one above and one
below, one in front and one in back. Each negative ion is surrounded in a similar
way by six positive ions. There is nothing that we can properly call a molecule of
sodium chloride. A particular sodium ion does not "belong" to any one chloride
ion; it is equally attracted to six chloride ions. The crystal is an extremely strong,
rigid structure, since the electrostatic forces holding each ion in position are
powerful. These powerful interionic forces are overcome only at. a very high
temperature; sodium chloride has a melting point of 801 "C.
Crystals of other ionic compounds resemble crystals of sodium chloride in
having an ionic lattice, although the exact geometric arrangement may be different.
As a result, these other ionic compounds, too, have high melting points. Many
molecules contain both ionic and covalent bonds. Potassium nitrate, KN03, for
example, is made up of K+ ions and NO3- ions; the oxygen and nitrogen atoms
of the NO3- ion are held to each other by covalent bonds. The physical properties
of compounds like these are largely determined by the ionic bonds; potassium
nitrate has very much the same sort of physical properties as sodium chloride.
A non-ionic compound, one whose atoms are held to each other entirely by
covalent bonds, forms crystals in which the structural units are molecules. It is theforces holding these molecules to each other that must be overcome for melting to
occur. In general, these intermolecular forces are very weak compared with theforces holding ions to each other. To melt sodium chloride we must supply enough
energy to break ionic bonds between Na+ and C1-. To melt methane, CH,, we
do not need to supply enough energy to break covalent bonds between carbon and
hydrogen; we need only supply enough energy to break CH, molecules away from
each other. In contrast to sodium chloride, methane melts at - 183 "C.
Intermolecular forces
What kinds of forces hold neutral molecules to each other? Like interionic
forces, these forces seem to be electrostatic in nature, involving attraction of
positive charge for negative charge. There are two kinds of intermolecular forces:
dipole-dipole interactions and van der Wauls forces.
Dipoldpole interaction is the attraction of the positive end of one polar
molecule for the negative end of another polar molecule. In hydrogen chloride, for
example, the relatively positive hydrogen of one molecule is attracted to the
relatively negative chlorine of anotherAs a result of dipole-dipole interaction, polar molecules are generally held to each
other more strongly than are non-polar molecules of comparable molecular weight;
this difference in strength of intermolecular forces is reflected in the physical
properties of the compounds concerned.
An especially strong kind of dipoledipole attraction is hydrogen bonding, in
which a hydrogen atom serves as a bridge between two electronegative atoms, holding
one by a covalent bond and the other by purely electrostatic forces. When hydrogen is
attached to a highly electronegative atom, the electron cloud is greatly distorted
toward the electronegative atom, exposing the hydrogen nucleus. The strong
positive charge of the thinly shielded hydrogen nucleus is strongly attracted by the
negative charge of the electronegative atom of a second molecule. This attraction
has a strength of about 5 kcal/mol, and is thus much weaker than the covalent
bond-about 50-100 kcal/mol-that holds it to the first electronegative atom. It isFor hydrogen bonding to be important, both electronegative atoms must come from
the group: F, 0, N. Only hydrogen bonded to one of these three elements is positive
enough, and only these three elements are negative enough, for the necessary
attraction to exist. These three elements owe their special effectiveness to the
concentrated negative charge on their small atoms.
There must be forces between the molecules of a non-polar compound, since
:ven such compounds can solidify. Such attractions are called van der Waals forces.
The existence of these forces is accounted for by quantum mechanics. We can
roughly visualize them arising in the following way. The average distribution of
charge about, say, a methane molecule is symmetrical, so that there is no net dipole
moment. However, the electrons move about, so that at any instant the distribution
will probably be distorted, and a small dipole will exist. This momentary
dipole will affect the electron distribution in a second methane molecule
nearby. The negative end of the dipole tends to repel electrons, and the positive
end tends to attract electrons; the dipole thus induces an oppositely oriented dipole
in the neighboring molecule : Although the momentary dipoles and induced dipoles are constantly changing, the
net result is attraction between the two molecules.
These van der Waals forces have a very short range; they act only between
the portions of different molecules that are in close contact, that is, between the surfaces
of molecules. As we shall see, the relationship between the strength of van
der Waals forces and the surface areas of molecules