BONDING
http://www.science.uwaterloo.ca/~cchieh/cact/c123/intermol.html
In general, intermolecular forces can be divided into several categories. The four
prominent types are:
1. Strong ionic attraction
Recall lattice energy and its relations to properties of solid. The more ionic, the
higher the lattice energy. Examine the following list and see if you can explain the
observed values by way of ionic attraction:
LiF, 1036; LiI, 737; KF, 821; MgF2, 2957 kJ/mol.
2. Intermediate dipole-dipole forces
Substances whose molecules have dipole moment have higher melting point or
boiling point than those of similar molecular mass, but their molecules have no
dipole moment.
3. Weak London dispersion forces or van der Waal's force These forces alway
operate in any substance. The force arisen from induced dipole and the interaction
is weaker than the dipole-dipole interaction. In general, the heavier the molecule,
the stronger the van der Waal's force of interaction. For example, the boiling
points of inert gases increase as their atomic masses increases due to stronger
Landon dispersion interactions.
4. Hydrogen bond
Certain substances such as H2O, HF, NH3 form hydrogen bonds, and the
formation of which affects properties (mp, bp, solubility) of substance. Other
compounds containing OH and NH2 groups also form hydrogen bonds. Molecules
of many organic compounds such as alcohols, acids, amines, and aminoacids
contain these groups, and thus hydrogen bonding plays a important role in
biological science.
5. Covalent bonding
Covalent is really intramolecular force rather than intermolecular force. It is
mentioned here, because some solids are formed due to covalent bonding. For
example, in diamond, silicon, quartz etc., the all atoms in the entire crystal are
linked together by covalent bonding. These solids are hard, brittle, and have high
melting points. Covalent bonding holds atoms tighter than ionic attraction.
6. Metallic bonding
Forces between atom in metallic solids belong to another category. Valence
electrons in metals are rampant. They are not restricted to certain atoms or bonds.
Rather they run freely in the entire solid, providing good conductivity for heat and
electric energy. These behaviour of electrons give special properties such as
ductility and mechanical strength to metals.
http://people.seas.harvard.edu/~jones/es154/lectures/lecture_2/covalent_bond/covalent_b
ond.html
The Ionic Bond: Ionic bonds are formed when there is a complete transfer of
electrons from one atom to another, resulting in two ions, one positively charged
and the other negatively charged. For example, when a sodium atom (Na) donates
the one electron in its outer valence shell to a chlorine (Cl) atom, which needs one
electron to fill its outer valence shell, NaCl (table salt) results. Ionic bonds are often
4-7 kcal/mol in strength. (source)
The Covalent Bond: Covalent Bonds are the strongest chemical bonds, and are formed by
the sharing of a pair of electrons. The energy of a typical single covalent bond is ~80
kilocalories per mole (kcal/mol). However, this bond energy can vary from ~50 kcal/mol
to ~110 kcal/mol depending on the elements involved. Once formed, covalent bonds
rarely break spontaneously. This is due to simple energetic considerations; the thermal
energy of a molecule at room temperature (298 K) is only ~0.6 kcal/mol, much lower
than the energy required to break a covalent bond. (source)
Strength of Covalent Bond vs Ionic Bond
Question Strength of Covalent Bond vs Ionic Bond
Apparently, the AP Bio exam "says" covalent bonds are stronger than
ionic
bonds. When I check bond energies, they seem to be in the same range.
Is one stronger?
-----------------------------------------------Alas! Ideally, the term "bond energy" should probably be deleted from
the chemical lexicon. Like a lot of terms in chemistry, the closer you
look at the definition of the term the more you have a list caveats,
exceptions, and footnotes.
First, there is no such thing as a purely covalent, or purely ionic
bond. There is a continuum of covalent and ionic character that we can
assign to any bond.
Second, it depends upon the the products of dissociation. For example,
the energy required to dissociate NaCl ----> Na+ + Cl- [all in the gas
phase]is about 1.5 eV greater than the energy required to dissociate it
into neutral atoms. So the "covalent" NaCl bond is weaker than the
"ionic" NaCl bond.
Third, bond energies are useful when comparing similar classes of
substances. Since some covalent bonds form multiple bonds, e.g. N2, and
O2, they are stronger than F2 which only forms a single bond, but is
that a fair comparison. I do not think so.
Common student misconception
When comparing ionic and covalent compounds, it is
often stated that the ionic bond is stronger than
the covalent bond after completing various tests
(fusibility, volatility, mp., hardness)
This is because students are confusing the inter
and intra molecular bonding forces.
In an ionic compound, the intermolecular bonding
forces between ions that form a lattice are the
strongest, but below macromolecular covalent
networks. The intermolecular forces at work for
covalent compounds when conducting these tests is
actually the much weaker van der Waals forces.
The intramolecular force of attraction between ions
is weaker than its covalent intramolecular force.