Chapter 6 – Ionic Compounds
Week 7, Lesson 1
Properties of Ionic Compounds
• Ionic compounds have common properties:
– The have high melting and boiling temperatures
– The are hard but brittle
– The do not conduct electricity in the solid state
– The will conduct electricity if they are melted or
dissolved in water.
What do these properties reveal about
the nature of the particles in ionic
compounds?
• To answer this questions, sodium chloride will be
used as the example.
• Sodium chlorides crystals are cubic, this indicates
that the particles have a regular arrangement.
• The physical properties of NaCl are very different
to those of metals, so chemists have deduced
that their arrangements in solid ionic compounds
are different from those present in metals.
Properties and Structural Features of
NaCl
Property of NaCl
High Melting Temperature
Hard, brittle crystals
Does not conduct electricity in
solid state
Conducts electricity in molten
state
What this tells you about the
structure
Forces between particles are
strong
Forces between particles are
strong
No free-moving charged
particles present in the solid
sodium chloride
Free-moving charged particles
present in molten sodium
chloride
Properties and particles cont…
• From these properties we can find that:
– The forces between the particles are strong
– There are no free moving electrons present in
solid sodium chloride, unlike in metals
– There are charged particles present but in the
solid state, they are not free to move.
– However, when the ionic compound melts the
particles are free to move.
Ionic Bonding Model
• Chemists believe that when metallic and non
metallic atoms react to form compounds, the
following steps occur:
– Metal atoms lose electrons to non-metallic atoms and so
become positively charged metal ions.
– Non-metal atoms gain electron to form negatively charged
non-metal ions.
– Large numbers of positive and negative ions formed in this
way then combine to form 3D lattices.
– The 3D lattice is held together strongly by electrostatic
forces of attraction between the positive and negative
ions.
– The electrostatic force of attraction is called ionic bonding
Ionic Bonding Model
• Two examples representing
part of the crystal lattice
of the ionic compound
sodium chloride.
Properties of Ionic Compounds
• High melting temperature
– To melt an ionic solid, energy must be provided to allow
the ions to break free and move.
• Hardness and Brittleness
- There are strong electrostatic forces of attraction (ionic
bonding) between ions, so a strong force is needed to disrupt
the crystal lattice. A lattice cannot be scratched easily, however,
if it is hit with a hammer and shatters it is said to be brittle.
- This is because the layers of ions will move relative to each
other due to the force of the blow. During this movement ions of
like charge will move next to each and the resulting repulsion
will cause the shattering.
Repulsion causing Shattering
Properties continued…
• Electrical Conductivity
– In the solid form, ions in
sodium chloride are held in the
crystal lattice and are not free
to move, so they do not conduct
electricity.
– When solids melt, ions are free
to move. The movement of these
charged particle to an electrode
completes the circuit.
Properties continued…
• Reaction of Metal with Non-Metals
– When metallic atoms react with non-metallic
atoms, the metallic atoms lose electrons to the
non-metallic atoms.
– This mean that both atoms will often achieve the
electronic configuration of the nearest noble gas.
Week 7, Lesson 2
Electron Transfer Diagrams
• When sodium and chlorine react together to
form sodium chloride, the electron that a
sodium atom loses to form a stable sodium
ion is gained by a chlorine atom to form a
stable chloride ion.
• In the diagram on the next slide, only the
main electron shells are shown to
demonstrate the electron transfer.
Sodium + Chlorine => Sodium
Chloride
Other Representations…
• Electron transfer between sodium and
chlorine can also be illustrated by the
following equations:
11Na
1s22s22p63s1
11Na
+
1s22s22p6
e17Cl
1s22s22p63s23p5
-
17Cl
1s22s22p63s23p6
Overview of the Formation of
Sodium Chloride
• There are several stages in the reaction
between sodium and chlorine to form solid
sodium chloride.
• These include:
– Chlorine molecules splitting into separate chlorine
atoms,
– Electrons being transferred from sodium atoms to
chlorine atoms forming positively charged sodium
ions and negatively charged chloride ions,
– Sodium and chloride ions combining to form a 3D
ionic lattice.
Magnesium Chloride
• Magnesium chloride is often dissolved in
natural mineral waters. If the ideas of electron
transfer are applied to the formation of this
compound then;
– A magnesium atom, electron configuration of
1s22s22p63s2, will have a stable outer shell if 2
electrons are removed. A Mg2+ ion is formed.
– A chlorine atom, electron configuration of
1s22s22p63s23p5, will have a stable outer shell if it
gains 1 electron. A Cl- ion is formed.
Magnesium Chloride cont…
• This means magnesium donates two electrons
to 2 different chlorine atoms.
• This means it has a chemical formula of MgCl2
Week 7, Lesson 3
Chemical Formulas
• Almost every compound in which a metal is
combined with a non-metal displays ionic
bonding.
• The formulas of simple ionic compounds, such as
sodium chloride can be predicted from the
electronic configuration of the atoms involved or
from their location on the periodic table.
• Other cases are more complex so chemists find it
useful to learn the charge on a number of
common ions so that the chemical formula can
be written quickly.
The Rules: Writing Formulas
• Simple Ions
– The positive ion is placed first in the formula and
the negative second
– Positive and negative ions are combined so that
the total number of positive charges is balanced
by the total number of negative charges.
– When there are two or more of a particular ion in
a compound, then in the chemical formula the
number is written as a subscript after the
chemical symbol.
Polyatomic Ions
• Some ions contain more than one atom. These
are called polyatomic ions.
• They include nitrate, NO32-, carbonate, CO32-,
hydroxide, OH- and ammonium NH4+.
• If more than one of these ions is required to
balance the charge in a compound then it is
placed in brackets with the required number
written as subscript after the brackets.
Ions of Elements with Different
Electrovalencies
• Some elements form ions with different
charges.
• For example, iron sometimes forms a 2+ ion
and sometimes a 3+ ion.
• In this situation you need to specify the
electrovalency when naming the compound.
• This is done by placing an electrovalency of
the ion immediately after the metal in the
name of the compound, ie, Iron(II) chloride
Week 7, Lesson 4
Uses of Ionic Compounds
• Marble, calcium carbonate is an ionic
compound used for its hardness and strength.
• It is almost insoluble in water so marble
building and statues last a long time.
• Rocks and clays are much more complex
materials, but they also owe useful properties
to the presence of strong ionic bonding.
• Calcium phosphate is another ionic compound
useful for its physical properties, it gives
strength to our bones.
Uses cont…
• Many ionic compounds dissolve in water.
• In this process the ions separate and move
freely in the solution.
• Such solutions will conduct electricity.
• Sodium hydrogen carbonate, or bicarbonate
of soda, is used in baking.
• This compound decomposes when heated and
produces the carbon dioxide which forms
bubbles in cakes making them rise.
Uses cont…
• Sodium hydrogen carbonate is more commonly
combined with another ionic compound, potassium
hydrogen tartrate, or cream of tartar, in baking
powder.
• It is also found in antacid products as well as some
soluble pain killers.
• Sodium fluoride and calcium fluoride are added to
domestic water supplies in some cities because the
fluoride ion that is released reduces tooth decay.
• Fluoride toothpastes contain another ionic
compound, sodium monofluorophosphate, as a
source of fluoride ions.
Uses cont…
• Ammonium sulfate and ammonium nitrate are
commonly found in fertilisers as they dissolve
in water and release nitrogen containing ions,
which can be absorbed by plants.