How to Revise from this Manual
Colour What the colour means
Normal text; questions
What to watch out for
Examples
Solutions to questions
Your own solutions
, ~ Basics ~
Find chemical formulae
1. View elements as ions. Criss-cross charge numbers.
2. For polyatomic (> 1 atom) ions, use bracket. If charge number is 1, no need to
write 1 or bracket.
3. Simplify if possible.
Identify charge numbers / valency (number of electrons gained or lost)
From group number in Periodic Table
Group I II III IV V VI VII 0
number
Valency 1 2 3 4 3 2 1 2 for helium ,
8 for the rest
Li+ Mg2+ Al3+ C P3- O2- F- He, Ar
,From Roman numerals
● Only for most transition metals which have variable oxidation states.
Examples Copper (II) Copper (I) Manganese (VI)
Valency 2 so Cu2+ 1 so Cu+ 6 so Mn6+
Transition metals with fixed valency and Silver Zinc Cadmium
hence, no Roman numerals
Valency 1 so Ag+ 2 so Zn2+ 2 so Cd2+
For polyatomic ions
Examples Ammonium Carbonate Hydroxide Nitrate Sulfate ion:
ion: NH4+ ion: CO32- ion: OH- ion: NO3- SO42-
Valency 1 2 1 1 2
Naming ionic compounds
● 1st word: Metallic element. 2nd word: non-metallic element ending with ‘ide’.
Elements Calcium, chlorine Iodine, lithium Sulfur, potassium
Ionic compound Calcium chloride Lithium iodide Potassium sulfide
- If metallic element is transition metal, (Roman numeral) often used to assign its
valency. Iron (II) oxide, lead (IV) chloride.
● Polyatomic ions won’t change name after combining with other elements. Sodium
carbonate, potassium sulfate.
Naming covalent compounds
● 1st element unchanged. 2nd element's name ends with ‘ide’.
● Can use prefix for 2nd element based on number of its atoms present.
Number of 1 2 3 4
atoms
Prefix mono di tri tetra
Examples Carbon Carbon Sulfur Carbon
monoxide: CO dioxide: CO2 trioxide: SO3 tetrachloride: CCl4
● Some covalent compounds have specific names. Ammonia: NH3, water: H2O.
, Balancing chemical equations
● Sum of atoms before and after reaction is equal.
● Recount number of atoms for each element on both sides of equation after
balancing every atom.
1. Balance non-H and non-O atoms. If got 2 or more non-H and non-O atoms,
balance them 1 by 1 in any order.
2. Balance O atoms.
3. Balance H atoms.
4. Reverse steps 2 and 3 if needed.
Balance Fe2 (SO4)3 + CsOH → Cs2SO4 + Fe(OH)3.
1. Balance Fe, Cs or S in no particular order. Let’s say Fe, then Cs, then S.
1) Fe2 (SO4)3 + CsOH → Cs2SO4 + 2 Fe(OH)3.
2) Fe2 (SO4)3 + 2 CsOH → Cs2SO4 + 2 Fe(OH)3.
3) Fe2 (SO4)3 + 2 CsOH → 3 Cs2SO4 + 2 Fe(OH)3. Notice how the Cs atoms are now
unbalanced. It’s normal to change numbers you have added. Thus, change the 2
beside CsOH to 6 by multiplying by 3.
4) Fe2 (SO4)3 + 6 CsOH → 3 Cs2SO4 + 2 Fe(OH)3.
2. Balance the O. In this case, there are already 18 O atoms before and after the
reaction.
3. Balance the H. In this case, there are already 6 H atoms before and after the
reaction.
Final answer: Fe2 (SO4)3 + 6 CsOH → 3 Cs2SO4 + 2 Fe(OH)3.
Writing balanced chemical equations
● Write state symbols to indicate physical states. Solid: (s), liquid: (l), gas: (g),
aqueous solution: (aq). State symbol of water is (l), not (aq) !
Writing balanced ionic equations
NaCl (aq) + AgNO3 (aq) → AgCl (s) + NaNO3 (aq).
1. Split ions ONLY for water-soluble ionic compounds which ionise / dissociate into
ions in water.
Na (aq) + Cl- (aq) + Ag+ (aq) + NO3- (aq) → AgCl (s) + Na+ (aq) + NO3- (aq).
+
2. Cancel out spectator ions (ions that remain the same in original states before and
after reaction).
Remove spectator ions NO3- and Na+.
Colour What the colour means
Normal text; questions
What to watch out for
Examples
Solutions to questions
Your own solutions
, ~ Basics ~
Find chemical formulae
1. View elements as ions. Criss-cross charge numbers.
2. For polyatomic (> 1 atom) ions, use bracket. If charge number is 1, no need to
write 1 or bracket.
3. Simplify if possible.
Identify charge numbers / valency (number of electrons gained or lost)
From group number in Periodic Table
Group I II III IV V VI VII 0
number
Valency 1 2 3 4 3 2 1 2 for helium ,
8 for the rest
Li+ Mg2+ Al3+ C P3- O2- F- He, Ar
,From Roman numerals
● Only for most transition metals which have variable oxidation states.
Examples Copper (II) Copper (I) Manganese (VI)
Valency 2 so Cu2+ 1 so Cu+ 6 so Mn6+
Transition metals with fixed valency and Silver Zinc Cadmium
hence, no Roman numerals
Valency 1 so Ag+ 2 so Zn2+ 2 so Cd2+
For polyatomic ions
Examples Ammonium Carbonate Hydroxide Nitrate Sulfate ion:
ion: NH4+ ion: CO32- ion: OH- ion: NO3- SO42-
Valency 1 2 1 1 2
Naming ionic compounds
● 1st word: Metallic element. 2nd word: non-metallic element ending with ‘ide’.
Elements Calcium, chlorine Iodine, lithium Sulfur, potassium
Ionic compound Calcium chloride Lithium iodide Potassium sulfide
- If metallic element is transition metal, (Roman numeral) often used to assign its
valency. Iron (II) oxide, lead (IV) chloride.
● Polyatomic ions won’t change name after combining with other elements. Sodium
carbonate, potassium sulfate.
Naming covalent compounds
● 1st element unchanged. 2nd element's name ends with ‘ide’.
● Can use prefix for 2nd element based on number of its atoms present.
Number of 1 2 3 4
atoms
Prefix mono di tri tetra
Examples Carbon Carbon Sulfur Carbon
monoxide: CO dioxide: CO2 trioxide: SO3 tetrachloride: CCl4
● Some covalent compounds have specific names. Ammonia: NH3, water: H2O.
, Balancing chemical equations
● Sum of atoms before and after reaction is equal.
● Recount number of atoms for each element on both sides of equation after
balancing every atom.
1. Balance non-H and non-O atoms. If got 2 or more non-H and non-O atoms,
balance them 1 by 1 in any order.
2. Balance O atoms.
3. Balance H atoms.
4. Reverse steps 2 and 3 if needed.
Balance Fe2 (SO4)3 + CsOH → Cs2SO4 + Fe(OH)3.
1. Balance Fe, Cs or S in no particular order. Let’s say Fe, then Cs, then S.
1) Fe2 (SO4)3 + CsOH → Cs2SO4 + 2 Fe(OH)3.
2) Fe2 (SO4)3 + 2 CsOH → Cs2SO4 + 2 Fe(OH)3.
3) Fe2 (SO4)3 + 2 CsOH → 3 Cs2SO4 + 2 Fe(OH)3. Notice how the Cs atoms are now
unbalanced. It’s normal to change numbers you have added. Thus, change the 2
beside CsOH to 6 by multiplying by 3.
4) Fe2 (SO4)3 + 6 CsOH → 3 Cs2SO4 + 2 Fe(OH)3.
2. Balance the O. In this case, there are already 18 O atoms before and after the
reaction.
3. Balance the H. In this case, there are already 6 H atoms before and after the
reaction.
Final answer: Fe2 (SO4)3 + 6 CsOH → 3 Cs2SO4 + 2 Fe(OH)3.
Writing balanced chemical equations
● Write state symbols to indicate physical states. Solid: (s), liquid: (l), gas: (g),
aqueous solution: (aq). State symbol of water is (l), not (aq) !
Writing balanced ionic equations
NaCl (aq) + AgNO3 (aq) → AgCl (s) + NaNO3 (aq).
1. Split ions ONLY for water-soluble ionic compounds which ionise / dissociate into
ions in water.
Na (aq) + Cl- (aq) + Ag+ (aq) + NO3- (aq) → AgCl (s) + Na+ (aq) + NO3- (aq).
+
2. Cancel out spectator ions (ions that remain the same in original states before and
after reaction).
Remove spectator ions NO3- and Na+.
