|Chemical bonds are the electrostatic force of attraction which holds
atoms, ions and molecules together. Clues to the type of bond can be obtained
by studying the properties of substances and the structural characteristics
of the particle. Understanding the nature and origin of chemical bonds is
an important part of understanding chemistry, because changes in these bonding
forces constitute the underlying basis for all chemical reactions. Old bonds
break and new bonds form when chemicals react.
|Ionic bonds are formed when metals react with non-metals. For example,
when sodium reacts with chlorine, the sodium loses one electron while the
chlorine gains one electron. The Na atom which was electrically neutral
takes on a positive charge and the chlorine atom which was also electrically
neutral takes on a negative charge.
|Nao -----> Na+ + e-
Clo + e- -----> Cl-
Nao + Cl- -> Na+Cl-
|The reason for the attraction is the fact that opposite charges
attract. But why are electrons transferred between these two atoms? Why does
Nao form Na+ ions and not Na2+ or Na-?
Why does Clo form Cl- and not Cl2- or Cl+
ions? The fact is it depends upon an energy change. In order for these ions
to form there is a net energy decrease to a more stable energy level. Making
a Na2+ ion is not possible because it would require to much energy.
The same is true for the Cl-. Making a Cl- ion is easy.
You would have to force it to become Cl2-.
|Three factors affect the energy involved in the formation of an
ionic compound. One is the removal of electrons from the atoms that become
cations (eg. sodium). Formation of a cation requires an input of energy -
the ionization energy. (The amount of energy it takes to move an electron
out of orbit in a neutral atom and remove it to some infinite point away
from the nucleus is the ionization energy). You have a table of ionization
energies in your databook. A second factor is the energy change that accompanies
the addition of one or more electrons to the atoms that become anions. (eg.
chlorine). This energy is the electron affinity. The ionization energy
and the electron affinity are energies associated with the changes of isolated
gaseous atoms. A crystal of salt, however, does not consist of isolated atoms.
A crystal of salt is a group of ions packed tightly into a regular pattern.
This pattern is referred to as a lattice, and it has a lower energy
than the isolated ions.
|To understand this, imagine that we want the vaporize a salt crystal.
In order to do this we must add heat energy in order to get the crystal
vibrating fast. In the crystal the forces of attraction exceed the forces
of repulsion, so to accomplish our vaporization we have to add enough energy
to overcome these forces of attraction. This would of course require work,
so vaporizing the crystal increases the ions' potential energy and is endothermic.
The reverse process - the imaginary process that forms the lattice form from
isolated ions - must therefore lead to a lowering of the potential energy
of the system and be exothermic. The amount that the energy of the system
is lowered because of these mutual attractions of its ions is the lattice
|The lattice energy is the major stabilizing factor for ionic compounds.
In almost every case, the energy input required by the ionization energy
is larger than the energy recovered by the electron affinity, so the IE and
EA combined have a net energy-raising effect. It is were not for the large
energy-lowering effect of the lattice energy, formation of ionic compounds
would be endothermic and they simply wouldn't be formed.
|Now why do atoms react? Right from the beginning, you where told
that metals tend to form positive ions and non-metals tend to form negative
ions. At the left of the period table are the metals - elements with small
IE and EA. Relatively little energy is needed to remove electrons from them
to produce positive ions. At the upper right of the periodic table are the
non-metals - elements with large IE and EA. It is very difficult to remove
electrons from these elements, but sizeable amounts of energy are released
when they gain electrons. On an energy basis, it is least "expensive" to
form a cation from a metal and an anion from a non-metal.
|Formation of Ions by the Representative Elements|
|What happens when sodium loses an electron. The electronic structure
of Na is
|The electron that is lost is the one least tightly held. For sodium
that is the single outer 3s electron. The electronic structure of the Na+
ion, then is
|The removal of the first electron from Na does not require much
energy because the first IE of Na is so small.
|For Na 1st IE = 496 kJ/mol
2nd IE = 4563 kJ/mol
|Therefore, an input of energy equal to the first IE can be easily
recovered by the exothermic lattice energy of ionic compounds containing
the Na+ ion. However, removal of a second electron from sodium
is very difficult - the second IE of Na is enormous. The amount of energy
that must be invested to create a Na2+ ion is therefore much greater
then the amount of energy that can be recovered by the lattice energy, so
overall the formation of a compound containing Na2+ is very energetically
unfavourable. This is why we never observe compounds that contain this ion,
and why sodium stops losing electrons once it has achieved a noble gas configuration.
|A similar situation exists for other metals too. Consider calcium,
for example. We known that this metal forms ions with a 2+ charge. This
means that when it reacts, a calcium atom loses its two outermost electrons.
|The two 4s electrons of Ca are not held too tightly, so the amount
of energy that must be invested to remove them (the sum of the first and
second IE) can be recovered easily by the lattice energy of a Ca2+
|For calcium 1st IE = 590 kJ/mol
2nd IE = 1140 kJ/mol
3rd IE = 4912 kJ/mol
|However, the removal of yet another electron from calcium to form
Ca3+ requires breaking into the noble gas core. A tremendous
amount of energy is needed to accomplish this - much more than would be regained
by the lattice energy of Ca3+ compound. Therefore, a calcium atom
loses just two electrons when it reacts.
|For sodium and calcium, we find that the stability of the noble
gas core that lies below the outer shell of electrons effectively limits
the number of electrons that they lose, and that the ions that are formed
have a noble gas electron configuration. A similar configuration also tends
to be the fate of nonmetals when they form anions.
|Chlorine and oxygen are typical non-metals that form anions when
they react with metals such as sodium or calcium. When a chlorine atom reacts,
it gains one electron. For chlorine we have
|and when an electron is gained, its configuration becomes
|At this point, electron gain ceases, because if another electron
were to be added, it would have to enter an orbital in the next higher shell.
|With oxygen, a similar situation exists. The formation of the oxide
ion, O2-, gives oxygen a noble gas configuration without much
|and the large lattice energies of metal oxides leads to stable compounds.
However, we never see the formation of O3- because, once again,
the last electron would have to enter an orbital in the next higher shell,
and this is very energetically unfavourable.
|The energy factors cause many atoms to form ions that have a noble
gas electron configuration. This leads us to the useful generalization that
when they form ions, atoms of most of the representative elements
tend to gain or lose electrons until they have obtained a configuration
that is that of the nearest noble gas.