Ionic Bonds and Ionic
Compounds
• Most of the rocks and minerals that make up the Earth’s crust are
composed of positive and negative ions held together by ionic
bonding. An ionic compound is an electrically neutral compound
consisting of positive and negative ions. You are very familiar with
some ionic compounds, such as sodium chloride (NaCl). A sodium
chloride crystal contains equal numbers of positive sodium ions (Na+)
and negative chloride ions (Cl−).
• Ionic Bonds
• Oppositely charged particles attract each other. This attractive force is
often referred to as an electrostatic force. An ionic bond is the
electrostatic force that holds ions together in an ionic compound. The
strength of an ionic bond is directly dependent upon the magnitudes of
the charges and inversely dependent on the distance between the charged
particles. For example, a cation with a 2+ charge will make a stronger
ionic bond than a cation with a 1+ charge.
• Additionally, a larger ion will form a weaker ionic bond than a smaller
ion, due to the larger distance between its nucleus and the electrons of
the oppositely charged ion.
• Video On Ionic Bonding
• 1.How does this animation represent the transfer of electrons?
• 2.How do the sodium chloride units join together?
• Electron Dot Diagrams
• We will use sodium chloride as an example to demonstrate the nature
of the ionic bond and how it forms. As you know, sodium is a metal,
and it can lose its one valence electron to become a cation. Chlorine is a
nonmetal, and it gains one electron to become an anion. By forming
ions in this way, both atoms achieve a noble gas electron configuration.
However, electrons cannot be simply “lost” to nowhere in particular, nor
can they be "gained" without a source.
• In the case of sodium chloride, a single electron is transferred from the
sodium atom to the chlorine atom, as shown below.
• The ionic bond is the attraction between the Na+ ion and the Cl− ion.
It is conventional to show the cation without any dots around the
symbol, since the energy level that originally contained the valence
electron(s) is now empty. The anion is now shown with a complete
octet of electrons.
• For a compound such as magnesium chloride, the two elements are not
combined in a 1:1 ratio. Because magnesium has two valence
electrons, it needs to lose both to achieve a noble gas configuration.
Since chlorine only has room for one more electron in its valence
level, two chlorine atoms must be present as electron acceptors in order
to form each Mg2+ ion.
• The final formula for magnesium chloride is MgCl2
• Formula Units
• The formula unit is the lowest whole number ratio of the ions present
in an ionic compound. The formula unit of sodium chloride is NaCl,
while the formula unit of magnesium chloride is MgCl2. The formula
unit of an ionic compound is always an empirical formula.
• Ionic Compounds
• The electron dot diagrams show the nature of the electron transfer that
takes place between metal and nonmetal atoms. However, ionic
compounds do not exist as discrete molecules, as the dot diagrams may
suggest. In order to minimize the potential energy of the system, as
nature prefers, ionic compounds take on the form of an extended threedimensional array of alternating cations and anions. This maximizes the
attractive forces between the oppositely charges ions.
• The figure below shows two different ways of representing the ionic
crystal lattice. A ball and stick model makes it easier to see how
individual ions are oriented with respect to one another. A space filling
diagram is a more accurate representation of how the ions pack
together in the crystal.
• Coordination Number
• The coordination number is the number of ions that immediately surround
an ion of the opposite charge within a crystal lattice. If you examine the
figure above you will see that there are six chloride ions immediately
surrounding a single sodium ion, so the coordination number of sodium is 6.
Likewise, six sodium ions immediately surround each chloride ion, making
the coordination number of chloride also equal to 6. Because the formula unit
of sodium chloride displays a 1:1 ratio between the ions, the coordination
numbers must be the same.
• The formula unit for cesium chloride is CsCl, also a 1:1 ratio.
However, as shown below, the coordination numbers are not 6, like
they are in NaCl. The center Cs+ ion is surrounded by eight Cl− ions
at the corners of the cube. Each Cl− ion is also surrounded by eight
Cs+ ions. The coordination numbers in this type of crystal are both 8.
CsCl and NaCl do not adopt identical crystal packing arrangements
because the Cs+ ion is considerably larger than the Na+ ion.
• Another type of crystal is illustrated by titanium(IV) oxide, TiO2,
which is commonly known as rutile. The rutile crystal is shown below.
• The gray Ti4+ ions are each surrounded by six red O2− ions. The O2−
ions are each surrounded by three Ti4+ ions. The coordination of the
titanium(IV) cation is 6, which is twice the coordination number of the
oxide anion, which is 3. This fits with the formula unit of TiO2, since
there are twice as many O2− ions as Ti4+ ions.
• The crystal structure of any ionic compound must reflect its formula
unit. For example, in a crystal of iron(III) chloride, FeCl3, there are
three times as many chloride ions as iron(III) ions.
• Physical Properties of Ionic Compounds
• Pictured below are a few examples of the color and brilliance of
naturally occurring ionic crystals.
• The regular and orderly arrangement of ions in the crystal lattice is
responsible for the various shapes of these crystals, while transition
metal ions give rise to the colors.
• Because of the many simultaneous attractions between cations and
anions that occur, ionic crystal lattices are very strong. The process of
melting an ionic compound requires the addition of large amounts of
energy in order to break all of the ionic bonds in the crystal. For
example, sodium chloride has a melting temperature of about 800°C.
• Ionic compounds are generally hard but brittle. Why? It takes a large
amount of mechanical force, such as striking a crystal with a hammer,
to force one layer of ions to shift relative to its neighbor. However,
when that happens, it brings ions of the same charge next to each
other. The repulsive forces between ions of the same charge causes the
crystal to shatter. When an ionic crystal breaks, it tends to do so along
smooth planes because of the regular arrangement of the ions.
• Another characteristic property of ionic compounds is their electrical
conductivity. The figure below. The picture shows three experiments
in which two electrodes that are connected to a light bulb are placed in
beakers containing three different substances.
• In the first beaker, distilled water does not conduct a current because
water is a molecular compound. In the second beaker, solid sodium
chloride also does not conduct a current. Despite being ionic and thus
composed of charges particles, the solid crystal lattice does not allow
the ions to move between the electrodes. Mobile charged particles are
required for the circuit to be complete and the light bulb to light up. In
the third beaker, the NaCl has been dissolved into the distilled water.
• Now the crystal lattice has been broken apart and the individual
positive and negative ions can move. Cations move to one electrode,
while anions move to the other, allowing electricity to flow. Melting an
ionic compound also frees the ions to conduct a current. Ionic
compounds conduct an electric current when melted or dissolved in
water.
• •One or more electrons are transferred from a metal atom to a
nonmetal atom to form ions. Ionic bonds are the electrostatic
attractions between positive and negative ions.
• •An ionic compound is a three-dimensional network of alternating
cations and anions that are mutually attracted to one another. The
coordination number of an ion is the number of nearest neighbors that it
has within the crystal lattice.
• •Ionic compounds are hard and have high melting points. They are
difficult to break but are also very brittle. They conduct electricity only
when melted or dissolved in water to form a solution.