The kind of questions that require the consideration of intramolecular attractive
forces require comparison of boiling points or melting points...phase changes.
Phase changes in elements and compounds.
Elements
There are four type of elements;
Atomic (He, Ne, Ar, Kr);
molecular (H2, N2, O2, F2,
Cl2, Br2, I2, P4, S8);
metallic (all metals);
extended covalent (C, Si);
For for atomic and molecular elements the bonding involved in phase changes
are dispersion forces. Since the atomic and molecular elements exist as
individual atoms or molecules the attractive force that must be overcome
are intermolecular/interatomic. For metallic elements the forces that must
be overcome are metallic. Covalent bonds must be broken in carbon and silicon.
For carbon and silicon the issue of melting does require breaking covalent
bonds, an intra-macromolecular force.
Compounds
There are two types of compounds;
Ionic (formula includes a metallic element and a nonmetallic element*);
Covalent (formula includes nonmetallic elements*);
For ionic compounds the attractive forces that must be overcome in melting
are ionic, the electrostatic attraction of ions. This force, between ions,
is an intra ionic type of attractive force. In an ionic compound the pure
substance is a 3-dimensional array of oppositely charged ions. For covalent
compounds the attractive forces that must be overcome are intermolecular
and are either hydrogen-bonding, dipole-dipole and dispersion forces. For
covalent compounds the smallest form of the pure substance is a molecule.
Separating covalent molecules involves breaking intermolecular attractive
forces.
*There are some exceptions to these general rules for using formulas to
predict the type of bonding that occurs in the compound.
The most important exception to the rule that ionic compounds in their
formula have a metallic element and a nonmetallic element, is the ammonium
ion, NH4+. The presence of the pattern NH4
in a formula is also a characteristic of an ionic compound. Additional
examples include other derivatives of ammonia (weak bases), including,
methyl ammounium ion (CH3NH3+), dimethyl
ammonium ((CH3)2NH2+), and
others.
There are some exceptions to the formula of covalent compounds. There
are examples of substances with formulas of metals and nonmetals which
are principly covalent. This fundamental is related to the size of the
cation. Most very small cations found in compounds show more covalent
compounding. Examples are BeCl2, and Al2Br6
as well as other halides of these two metals. Magnesium show some covalent
character also, but not to the same extend that beryllium does.
SiO2 is the only example of an extended covalent interaction
in a compound. This substance has one of the highest melting points.
The distinction between intra- and inter- molecular attractive forces
is important. Student must know that for covalent compounds, phase changes
occur when intermolecular attractive forces are broken. Phase changes do
not occur when intramolecular attractive forces are broken. Students must
be sure to clearly state, or imply that it the attraction between molecules
that are overcome when a phase change occurs.
Strengths of Intra- and intermolecular attractive forces
The strength of ionic bonds, which must overcome when ionic compounds melt,
depend directly on the magnitude of the charge on the cation and anion, and
inversely on the size of the ions. Charge is significantly more important
than size when determining the stregth of ionic bonds.
The relative strength of intermolecular attractive forces in covalent compounds
must be carefully compared. When considering compounds with elements in the
first and second period only the order of strength is hydrogen-bonding >
dipole-dipole > London dispersion forces. Remember dispersion forces are
present in all substances. When we move to compounds containing elements from
the third period or higher, we no longer have hydrogen-bonding occurring,
only dipole-dipole forces (polar compounds) and London dispersion forces.
In these compounds dispersion forces are generally the most important. So
the more electrons in the compound, the more polarizable the compound is,
the stronger its dispersion forces.
When comparing elements extend covalent attractive forces > metallic
> dispersion forces. It is unlikely that students will ever have to argue
which metal has a stronger attractive force. However, comparing extended covalent
(C or Si) to dispersion forces (F2, Cl2, Br2)
is likely. Extended covalent are alway stronger than dispersion forces! Just
characterizing the presence of either and stating that extend covalent is
stronger compared to dispersion is sufficient. The trend in strength of dispersion
forces depends on the the number of electrons, the polarizability, of the
element.