1. STANDARD GRADE CHEMISTRY
Writing Formulae.
Formulae of compounds can be written using either
1. The name of the compound.
“mono” means one
“di” means two
“tri” means three
“tetra” means four
“penta” means five
If any of these prefixes appears in the name of the compound we use this to write the formula.
e.g. carbon monoxide is CO carbon dioxide is CO2

2. e.g. In iron(III) oxide, the iron has a valency of 3.
Using Valency Rules
If the name does not give any clues about the formula, we use valency rules.
e.g. sodium oxide
write symbols Na O
write valency 1 2
cross over valency 2 1
cancel if necessary
write formula Na2O
Note:- For transition metals the valency is usually written as a Roman numeral in brackets after the name of the transition metal
e.g. In iron(III) oxide, the iron has a valency of 3.
Standard Grade Chemistry

3. ·   3. Formulae involving Complex (group) Ions
If the name of the compound ends in “-ate” or “ –ite” , it contains a complex ion (watch also for hydroxide). Complex ions are found in your data book (p4).
e.g. Copper(II) nitrate
write symbols or formula Cu NO3-
write valency
(for complex ions, this
is the charge)
cross over valency
Use brackets and cancel
if necessary
write formula Cu(NO3)2

4. ·   4. Formulae Showing Charges
Sometimes you are asked to show the formula with charges (ionic formula)
e.g. sodium carbonate
write symbols or formula Na CO32-
write valency
(for complex ions, this
is the charge)
cross over valency
Use brackets and cancel
if necessary
write formula leaving charges (Na+ ) 2 CO32-
Write ionic formulae for the compounds listed above.

5. Calculations for you to try.
1. Write the formula for:-
Sulphur trioxide
Dinitrogen monoxide
Phosphorus pentachloride
(d) Carbon terachloride.
SO3
N2O
PCl5
CCl4
2. Write the formula for:-
Sodium oxide.
Aluminium iodide
Iron(III) oxide
(d) Silver(I) oxide.
Na2O
AlI3
Fe2O3
Ag2O
3. Write the formula for:-
Sodium carbonate
Aluminium nitrate
Iron(III) hydroxide
(d) Nickel(II) sulphate
Na2CO3
Al(NO3)3
Fe(OH)3
Standard Grade Chemistry
NiSO4

6. 4. Write the ionic formula for:-
Lithium hydrogencarbonate
Aluminium hydroxide
Iron(II) sulphate
(d) Copper(II) nitrate.
Li+HCO3-
Al3+(OH-)3
Fe2+SO42-
Cu2+(NO3-)2