1. Naming Ionic Compounds
Science 10 Unit 2

2. Lowest common multiple of 2 and 3 is 6
1. Writing Formulas
In an ionic compound, positive charges must balance the negative charges.
e.g.: What is the formula for magnesium phosphide?
Magnesium is Mg2+
Phosphorous is P3-
Lowest common multiple of 2 and 3 is 6
3 Mg2+ ions and 2 P3– ions

3. Mg3P2 CaO Mg2+ P3- Mg2+ P3- Mg2+ 6+ 6-
e.g.: What is the formula for calcium oxide?
Calcium is Ca2+
Oxygen is O2-
1 Ca2+ ion and 1 O2– ion
CaO
(remember, the subscript 1 is never written)

4. Only use if you understand that the + and - charges must balance!
Short-cut
Only use if you understand that the + and - charges must balance!
Magnesium oxide
1. Write both symbols (+ion first) with ion charge 2+ 2- Mg O
2. Criss-cross the charges and write them as subscripts
Mg O 2 2
MgO
3. Reduce if possible
(remember, the subscript 1 is never written)

5. Sometimes, metal ions have more than one combining capacity
e.g.
Cu2+
Cu1+
Fe3+
Fe2+
The name of the compound will include a roman numeral that tells you which combining capacity to use 1+ I 5+ V 2+ II 6+ VI 3+
III 7+
VII 4+ IV
e.g.
Iron II oxide
Iron III oxide 2+ 2- 3+ 2- Fe O Fe O
FeO
Fe2O3

6. 2. Naming Cu Cu 1. Write the name of the metal first.
2. Write the name of the non-metal second,
and change the ending to “-ide”
3. ONLY IF THE METAL HAS 2 OR MORE ION CHARGES,
Include a roman numeral to indicate which one it is.
e.g.
NaCl
Sodium
chloride
MgF2
Magnesium
fluoride 1+ Cu
CuCl2
copper has 2 ions: 2+ Cu
which one is it?

7. To find out which copper ion it is, look at the charge on the anion and balance the charges in reverse.
CuCl2 2+ 1- Cu Cl
from periodic table 1- Cl 2-
Therefore it is copper II chloride
CuCl therefore must be copper I chloride

8. Polyatomic Ions K PO4 K3PO4
Some groups of atoms act as individual ions
Ion Charge
e.g.
SO4-2
Sulphate ion -2
OH-1
Hydroxide ion -1
PO4-3
Phosphate ion -3
When writing formulas and naming, pretend the polyatomic ion is just one atom and follow the rules as usual. +1 -3
e.g. K
PO4
Potassium phosphate
K3PO4
Don’t change the endings of polyatomic ions

9. More Examples! Al SO4 Al2(SO4)3 Cu NO3 CuNO3 Aluminum sulphate+3 -2 Al
SO4
Al2(SO4)3
Aluminum sulphate
Put brackets around the SO4 to indicate 3 sulphate ions, not 43 oxygen atoms +1 -1 Cu
NO3
CuNO3
copper I nitrate
Don’t forget the roman numeral since copper has more than one combining capacity