Ionic bonding occurs when a metal atom transfers electrons to a non-metal atom to form ions. The ions have full outer electron shells, making them stable. The positively charged metal ions are attracted to the negatively charged non-metal ions, forming an ionic bond. To determine the formula of an ionic compound, you write the symbols of the elements, determine the charge of each ion, and balance the ions so the total charge is zero. This gives the ratio of ions used to write the chemical formula. For example, sodium oxide has a 2:1 ratio of sodium and oxide ions, so its formula is Na2O.

1. © Boardworks Ltd 20141 of 10

2. © Boardworks Ltd 20142 of 10
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.

3. © Boardworks Ltd 20143 of 10
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.
Chlorine has 7 electrons
in its outer shell.
2.8.7 [2.8.8]-
+
Cl Cl
-
2.8.1 [2.8]+
Na Na
By losing this electron,
it has a filled outer shell
and forms a positive ion.
By gaining an electron
from sodium, it has a
filled outer shell and
forms a negative ion.

4. © Boardworks Ltd 20144 of 10
How are ionic bonds formed?
The positive sodium ions and the negative chloride ions
are strongly attracted to each other.
-
Cl
It is this electrostatic attraction that forms ionic bonds in
sodium chloride and other ionic compounds.
Na
+

5. © Boardworks Ltd 20145 of 10
Ions and ionic bonding – summary

6. © Boardworks Ltd 20146 of 10
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
O O2-
Na+
Na+
1 electron
from each
Na atom
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.

7. © Boardworks Ltd 20147 of 10
How to write the formula of an ionic compound
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.
The formula of a compound uses chemical symbols and
numbers to show the ratio of atoms of each element present.

8. © Boardworks Ltd 20148 of 10
Formula of aluminium bromide
What is the formula of aluminium bromide?
Al Br
+3 -1
AlBr3
3 bromide ions are needed
for each aluminium ion
1 : 3
Symbol
Balance the
number of ions
Ion charge
Ratio of ions
Formula
Al
Br
Al3+
Br
Br
Br
-
Br
-
Br
-
1 electron
for each
bromine
atom

9. © Boardworks Ltd 20149 of 10
What is the ionic formula?

10. © Boardworks Ltd 201410 of 10
Want to see more?
This is only a sample of one of thousands of
Boardworks Science presentations.
To see more of what Boardworks can offer, order a
full presentation completely free:
www.boardworks.co.uk/internationalsciencepresentation