SME
Grade 10 Science · Problem type
The formula of an ionic compound
Use this to find an ionic compound’s formula from the charges of its ions: Mg²⁺ with Cl⁻ is MgCl₂, Al³⁺ with O²⁻ is Al₂O₃.
Diagram
Charges 2 and 1: Mg²⁺ with Cl⁻.
Each magnesium atom gives 2 electrons (lit); each chlorine atom takes 1 to fill its octet.
1 × 2 = 2 × 1 = 2 electrons move; the smallest whole-number ratio gives MgCl₂.
The ions’ opposite charges hold them together: an ionic bond.
This is an example. Choose your own values to start a new problem.
c₊
Charge of the metal ion
example
c₋
Size of the nonmetal ion’s charge
example
t
Electrons moved
worked out (not typed)
a
Metal ions in the formula
worked out (not typed)
b
Nonmetal ions in the formula
worked out (not typed)
Formulas
t
=
least
common
multiple
of
c₊
and
c₋
2 = least common multiple of 2 and 1
a
=
t
/c₊
1
=
2
2
b
=
t
/c₋
2
=
2
1
Assumptions
•
The metal gives electrons and the nonmetal takes them: the total positive charge equals the total negative charge.
•
The formula uses the lowest whole-number ratio of ions.
•
The picture draws Na⁺, Mg²⁺ or Al³⁺ for a charge of 1, 2 or 3, and Cl⁻, O²⁻ or N³⁻ for the nonmetal.
Step-by-step
WE KNOW
c₊ = 2, c₋ = 1
FIND
electrons moved (t), metal ions in the formula (a), nonmetal ions in the formula (b)
Step 1 · Find electrons moved (t)
Use t = least common multiple of c₊ and c₋
The electrons given must equal the electrons taken: the smallest number both charges go into.
t
=
least
common
multiple
of
2
and
1
t
=
2
Step 2 · Find metal ions in the formula (a)
Use a = t/c₊
Each metal ion gives as many electrons as its charge.
a
=
2
2
a
=
1
Step 3 · Find nonmetal ions in the formula (b)
Use b = t/c₋
Each nonmetal ion takes as many electrons as its charge.
b
=
2
1
The formula is MX₂, as in MgCl₂
b
=
2
Check
Put the numbers back in. Both sides should match.
2 = least common multiple of 2 and 1 ✓
1
=
2
2
✓
2
=
2
1
✓
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Ionic, covalent and metallic bonding
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Metallic bonding: a sea of electrons
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