Ions and attraction
Why do some ions attract while others repel?
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What is ionic bonding?
Compounds that contain ions are called ionic
compounds. These compounds are usually
formed by a reaction between a metal and a
non-metal.
Why do these substances react together and form bonds?
The metal and non-metal atoms have incomplete outer
electron shells and so are unstable.
Electrons are transferred from each metal atom to each
non-metal atom. The metal and the non-metal atoms form
ions with completely full outer shells and become stable.
The positive and negative ions are strongly attracted to each
other. This electrostatic attraction is called ionic bonding.
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How are ionic bonds formed?
Sodium chloride is an ionic compound formed by the reaction
between the metal sodium and the non-metal chlorine.
Sodium has 1 electron
in its outer shell.
By losing this electron,
it has a filled outer shell
and forms a positive ion.
+
Na
Na
2.8.1
[2.8]+
Chlorine has 7 electrons
in its outer shell.
By gaining an electron
from sodium, it has a
filled outer shell and
forms a negative ion.
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Cl
2.8.7
Cl
[2.8.8]© Boardworks Ltd 2007
How are ionic bonds formed?
The positive sodium ions and the negative chloride ions
are strongly attracted to each other.
-
+
Na
Cl
It is this electrostatic attraction that forms ionic bonds in
sodium chloride and other ionic compounds.
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Formation of an ionic bond
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Ions and ionic bonding – summary
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What is the ratio of ions?
Sodium chloride (NaCl) and magnesium oxide (MgO) are
simple ionic compounds.
In each compound, the metal needs to lose the same
number of electrons that the non-metal needs to gain.
1 electron
Cl
Na+
Cl-
Mg 2 electrons
O
Mg2+
O2-
Na
Both compounds have a 1:1 ratio of metal ions to non-metal
ions, which is shown by the formula of each compound.
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What is the ratio of ions in sodium oxide?
What is the ratio of ions needed to make sodium oxide?
Sodium (2.8.1) needs to lose 1 electron to form a sodium ion
but oxygen (2.6) must gain 2 electrons to form an oxide ion .
Na+
Na
Na
1 electron
from each
Na atom
O2-
O
Na+
Two sodium atoms are required for each oxygen atom
and so the ratio of sodium ions to oxide ions is 2:1.
From this ratio, the formula of sodium oxide is Na2O.
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What is the ratio of ions in magnesium chloride?
What is the ratio of ions needed for magnesium chloride?
Magnesium (2.8.2) needs to lose 2 electrons to form a
magnesium ion but chlorine (2.8.7) needs to gain 1 electron
to form an chloride ion.
Cl
Mg
1 electron
for each
Cl atom
ClMg2+
Cl
Cl-
Two chlorine atoms are required for each magnesium atom
and so the ratio of magnesium ions to chloride ions is 1:2.
From this ratio, the formula of magnesium chloride is MgCl2.
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How to write the formula of an ionic compound
The formula of a compound uses chemical symbols and
numbers to show the ratio of atoms of each element present.
To work out the formula of an ionic compound:
1. Write down the symbol for each element
– the metal is always written first.
2. Calculate the charge for each type of ion.
3. Balance the number of ions so that the positive
and negative charges are balanced and equal zero.
This gives the ratio of ions.
4. Use the ratio to write down the formula of the ionic
compound.
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Formula of aluminium bromide
What is the formula of aluminium bromide?
Symbol
Al
Br
Ion charge
+3
-1
Balance the
number of ions
3 bromide ions are needed
for each aluminium ion
Ratio of ions
1:3
Formula
AlBr3
Al
1 electron
for each
bromine
atom
Br
Br
Br
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Br
Al3+
-
BrBr© Boardworks Ltd 2007
Formula of aluminium oxide
What is the formula of aluminium oxide?
Symbol
Al
O
Ion charge
+3
-2
Balance the
number of ions
2 aluminium ions are
needed for 3 oxide ions
Ratio of ions
2:3
Formula
Al
2 electrons for
each oxygen atom
Al
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Al2O3
O
Al3+
O2O2-
O
Al3+
O
O2© Boardworks Ltd 2007
More ionic formulae
What are the formulae of all the possible ionic compounds
from combinations of these metals and non-metals.
metals
nonLi
Ca
Na
Mg
Al
K
metals
F
LiF
CaF2
NaF
MgF2
AlF3
KF
O
Li2O
CaO
Na2O
MgO
Al2O3
K2O
N
Li3N
Ca3N2
Na3N
Mg3N2
AlN
K3N
Br
LiBr
CaBr2
NaBr
MgBr2
AlBr3
KBr
S
Li2S
CaS
Na2S
MgS
Al2S3
K2S
Cl
LiCl
CaCl2
NaCl
MgCl2
AlCl3
KCl
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What is the ionic formula?
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What is an ionic lattice?
In an ionic compound, millions and millions of ions are
packed together in a regular cubic arrangement, joined by
ionic bonds.
This forms a giant 3D
structure called an
ionic lattice.
The ionic lattice will
continue to build in this way
until there are no more ions
left to add.
The structure of the ionic
lattice affects the properties
of the ionic compound.
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Why do ionic compounds form crystals?
Ionic compounds such as sodium chloride, form crystals, with
a cubic shape. This is due to the structure of the ionic lattice.
All ionic compounds form lattices and crystals when solid.
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Heating ionic compounds
Why are ionic compounds solid at room temperature and
have high melting points and boiling points?
Compound
Ion
charges
Melting
point (oC)
sodium chloride
1+ and 1-
801
1,413
magnesium oxide
2+ and 2-
2,852
3,600
Ionic bonds are strong and a
lot of heat is needed to break
them.
Boiling
point (oC)
strong
ionic bonds
hold ions
together
Larger ionic charges produce stronger ionic bonds and so
much more heat is required to break the ionic bonds in
magnesium oxide than in sodium chloride.
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Do ionic compounds conduct electricity?
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How can ionic compounds conduct electricity?
As solids, ionic compounds
cannot conduct electricity
because their ions are
bonded together in the lattice.
ions in
solid state
cannot
move
When liquid (molten), the ions
can break free of the lattice
and are able to move. The ions
are charged particles and so
can carry an electric current.
ions in
molten state
can move
and conduct
electricity
Ionic compounds are usually soluble in water because water
molecules have a slight electrical charge and so can attract
the ions away from the lattice. When dissolved, the ions are
free to move and can carry an electric current.
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Why are ionic compounds brittle?
Ionic compounds are brittle – they shatter when they are hit.
Ionic bonds are strong, so why does this happen?
force
repulsion
When the lattice is hit, a layer
of ions is shifted so that ions
with the same charges are
lined up together.
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These like charges repel
each other and so split
the ionic lattice causing
it to shatter.
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