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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
Cl-
Mg2+
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
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Br-
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+
O2-
O2-
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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