Chemical Bonding
Chemical compounds are formed by the joining of two or more atoms. A
stable compound occurs when the total energy of the combination has lower
energy than the separated atoms. The bound state implies a net attractive
force between the atoms ... a chemical bond. The two extreme cases of
chemical bonds are:
Covalent bond: bond in which one or more pairs of electrons are shared by
two atoms.
Index
Ionic bond: bond in which one or more electrons from one atom are removed
and attached to another atom, resulting in positive and negative ions which
attract each other.
Other types of bonds include metallic bonds and hydrogen bonding. The
attractive forces between molecules in a liquid can be characterized as van
der Waals bonds.
Bond
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Bond
data
Chemical
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Sodium chloride
Ionic
Hydrogen molecule
Covalent
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Covalent Bonds
Covalent chemical bonds involve the sharing of a pair of valence electrons by
two atoms, in contrast to the transfer of electrons in ionic bonds. Such bonds
lead to stable molecules if they share electrons in such a way as to create a
noble gas configuration for each atom.
Hydrogen gas forms the simplest covalent bond in the diatomic hydrogen
molecule. The halogens such as chlorine also exist as diatomic gases by
forming covalent bonds. The nitrogen and oxygen which makes up the bulk
of the atmosphere also exhibits covalent bonding in forming diatomic
molecules.
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Bond
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Chemical
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Covalent bonding can be
visualized with the aid of
Lewis diagrams.
Comparison of ionic and covalent materials.
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Polar Covalent Bonds
Covalent bonds in which the sharing of the electron pair is unequal, with the
electrons spending more time around the more nonmetallic atom, are called
polar covalent bonds. In such a bond there is a charge separation with one
atom being slightly more positive and the other more negative, i.e., the bond
will produce a dipole moment. The ability of an atom to attract electrons in
the presense of another atom is a measurable property called
electronegativity.
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Ionic Bonds
In chemical bonds, atoms can either transfer or share their valence electrons.
In the extreme case where one or more atoms lose electrons and other atoms
gain them in order to produce a noble gas electron configuration, the bond is
called an ionic bond.
Typical of ionic bonds are those in the alkali halides such as sodium chloride,
NaCl.
Index
Bond
concepts
Ionic bonding can
be visualized with
the aid of Lewis
diagrams.
Comparison of ionic and covalent materials.
Energy contributions to ionic bonds
Table of ionic diatomic bonds
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Metallic Bonds
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The properties of metals suggest that their atoms possess strong bonds, yet the
Bond
ease of conduction of heat and electricity suggest that electrons can move
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freely in all directions in a metal. The general observations give rise to a
picture of "positive ions in a sea of electrons" to describe metallic bonding.
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Metal Properties
The general properties of metals include malleability and ductility and most
Index
are strong and durable. They are good conductors of heat and electricity.
Their strength indicates that the atoms are difficult to separate, but
Bond
malleability and ductility suggest that the atoms are relatively easy to move in
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various directions. The electrical conductivity suggests that it is easy to move
electrons in any direction in these materials. The thermal conductivity also
involves the motion of electrons. All of these properties suggest the nature of
the metallic bonds between atoms.
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Hydrogen Bonding
Hydrogen bonding differs from other uses of the word "bond" since it is a
force of attraction between a hydrogen atom in one molecule and a small
atom of high electronegativity in another molecule. That is, it is an
intermolecular force, not an intramolecular force as in the common use of the
word bond.
When hydrogen atoms are joined in a polar covalent bond with a small atom
of high electronegativity such as O, F or N, the partial positive charge on the
hydrogen is highly concentrated because of its small size. If the hydrogen is
close to another oxygen, fluorine or nitrogen in another molecule, then there
is a force of attraction termed a dipole-dipole interaction. This attraction or
"hydrogen bond" can have about 5% to 10% of the strength of a covalent
bond.
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Bond
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Hydrogen bonding has a very important effect on the properties of water and
ice. Hydrogen bonding is also very important in proteins and nucleic acids
and therefore in life processes. The "unzipping" of DNA is a breaking of
hydrogen bonds which help hold the two strands of the double helix together.
Van der Waals Forces
It is important to remember that van der Waals' forces are forces that exist between
MOLECULES of the same substance. They are quite different from the forces that make
up the molecule. For example, a water molecule is made up of hydrogen and oxygen,
which are bonded together by the sharing of electrons. These electrostatic forces that
keep a molecule intact are existent in covalent and ionic bonding but they are NOT van
der Waals' forces.
The van der Waals' forces are the forces that exist between the millions of separate water
molecules, and not between the hydrogen and oxygen atoms in the case of water.
Dipole-Dipole forces are one of van der Waals' three forces. Dipole Dipole forces occur
in polar molecules, that is, molecules that have an unequal sharing of electrons. For
example, HCl comprised of the atom Hydrogen and Chlorine is polar. The Chlorine atom
has an extra electron, which came from the hydrogen atom. Because of this, the chlorine
part of the molecule is negatively charged, and the hydrogen side of the molecule is
positively charged.
ie. H - Cl
+ So in a solution where there are thousands of these molecules around that are slightly
charged on each side, the molecules naturally orient themselves the accommodate the
charge. The positive part of one molecule will move until it is next to the negative part of
a neighboring molecule. These forces between molecules tend to make them 'stick'
together.
Dispersion forces are another of van der Waals' three forces. They exist between
nonpolar molecules. For example, chlorine gas is made up of two chlorine atoms. In this
bond, the electrons are equally shared and are not dominant on one side of the molecule
as is the case in HCl. The atom looks like this
Cl - Cl
no overall charge on either side but, it is important to remember that within a bond,
electrons are constantly MOVING. They zoom around the atoms really quickly. As a
result, there may be a tiny instant in time where the electrons happen to be dominant on
one side, creating a situation like this,
Cl - Cl
+ However, this temporary charge disappears as quickly as it appeared because the
electrons are moving so fast. These temporary dipoles allow the temporarily negative side
of one molecule to attract the temporarily positive side of another molecule, which is the
intermolecular force.
Hydrogen bonding is the third type of van der Waals' forces. It is exactly the same as
dipole-dipole interaction, it just gets a special name. A hydrogen bond is a dipole dipole
interaction that occurs between any molecule with a bond between a hydrogen atom and
any of oxygen/fluorine/nitrogen. So, Hydrogen Fluoride (HF), Water (H2O), ammonia
(NH3)....any kind of substance that has a hydrogen bonded to either an oxygen, fluorine
or nitrogen atom, exhibits hydrogen bonding. The hydrogen bond is just the dipole dipole
force but it is extremely strong compared to either dipole dipole forces like HCl. It is
extremely strong because F N and O are extremely good at attracting electrons and H is
extremely good at losing them. So basically, the bond is EXTREMELY a one-sided
affair, resulting in an extreme dipole situation, thus named, a hydrogen bond. The
extremely positive side of the molecule will orient itself with the extremely negative side
of another molecule.
The van der Waals' forces are very weak. I said the hydrogen bond is extremely strong,
but that is only compared to the other van der Waals' forces. Compared to say, a covalent
bond, a hydrogen bond is approximately one tenth of that strength. The dipole-dipole
bond is weaker still, and the dispersion forces are the weakest of Van De Waals' forces.
That is demonstrated in the fact that, take for example, Cl2. Chlorine gas exhibits
dispersion forces, the weakest of van der Waals' forces. Cl2 is a GASEOUS compound,
because the dispersion forces are not strong enough to pull the molecules together as a
solid. The dispersion forces can only suffice to keep the substance as a gas, because the
forces between molecules are so weak that they can float about all over the place and
exist as a gas.
Now that we know what van der Waals' forces are we move on to the second part of the
question.
When you vaporize water, you need to turn it from a liquid to a gas. To do this, you need
to overcome the forces between the molecules, to allow them to float about freely by
themselves. You supply the substance with energy in the form of heat. The heat makes
the molecules vibrate. If the vibrations are strong enough, the molecules break free of the
van der Waals' forces that hold them together. In the case of water, these forces are
hydrogen bonding. So you supply the system with enough energy, and it is able to
overcome the forces between the molecules.
This accounts for the fact that water has an unusually high boiling point. Because
hydrogen bonds are stronger than the other van der Waals' forces, then water will take
more energy to overcome these bonds than say, HCl as a liquid- which would need
sufficiently less because it has weaker bonds between molecules to overcome.
So, in a sentence, the role of energy in vaporizing water is that is needed to overcome the
van der Waals' forces at work between the molecules.