Writing Chemical Formulae

The chemical formula of a compound shows what it's made of. It uses symbols to represent elements and their quantities. Let’s break it down!

Valency

Valency is the combining power of an element. It helps us know how elements connect with each other. Think of it as having "arms" to hold other atoms.

For example, imagine an octopus (O) with eight arms. This octopus can hold atoms with two arms each, like humans (H). The combination gives us OH₄, showing the octopus is holding four humans.

Here’s a quick

to some common ions and their valencies:

Valency	Name of Ion	Symbol	Non-metallic Ion	Symbol	Polyatomic Ion	Symbol
1	Sodium	Na⁺	Hydrogen	H⁺	Ammonium	NH₄⁺
	Potassium	K⁺	Hydride	H⁻	Hydroxide	OH⁻
	Silver	Ag⁺	Chloride	Cl⁻	Nitrate	NO₃⁻
2	Magnesium	Mg²⁺	Oxide	O²⁻	Carbonate	CO₃²⁻
	Calcium	Ca²⁺	Sulphide	S²⁻	Sulphate	SO₄²⁻
3	Aluminium	Al³⁺	Nitride	N³⁻	Phosphate	PO₄³⁻

Rules for Writing Chemical Formulae

1. Balance Valencies/Charges: The charges on the ions must match.
2. Order of Elements: The metal's symbol comes first.
   • Example: Calcium oxide is written as CaO, not OCa.
3. Polyatomic Ions: If there’s more than one, put it in brackets.
   • Example: Mg(OH)₂ for magnesium hydroxide.

Formulae of Simple Compounds

Binary compounds have two elements. To write their formulas:

• Write the elements and their valencies.
• Crossover their valencies to balance the formula.

Examples

1. Hydrogen Chloride

   • Formula: HCl

2. Hydrogen Sulphide

   • Formula: H₂S

3. Carbon Tetrachloride

   • Formula: CCl₄

4. Magnesium Chloride

   • Magnesium (Mg²⁺) and Chloride (Cl⁻) ions are crossed.
   • Formula: MgCl₂

5. Aluminium Oxide

   • The formula becomes Al₂O₃ as the valencies are the same.

6. Sodium Nitrate

   • Formula: NaNO₃

7. Calcium Hydroxide

   • Formula: Ca(OH)₂

8. Sodium Carbonate

   • Formula: Na₂CO₃

9. Ammonium Sulphate

   • Formula: (NH₄)₂SO₄

Exercises

1. Write down the formulae of:

   • (i) Sodium oxide: Na₂O
   • (ii) Aluminium chloride: AlCl₃
   • (iii) Sodium sulphide: Na₂S
   • (iv) Magnesium hydroxide: Mg(OH)₂

2. Name the compounds for these formulas:

   • (i) Al₂(SO₄)₃: Aluminium sulphate
   • (ii) CaCl₂: Calcium chloride
   • (iii) K₂SO₄: Potassium sulphate
   • (iv) KNO₃: Potassium nitrate
   • (v) CaCO₃: Calcium carbonate

3. What does chemical formula mean?
   A chemical formula represents the elements and their amounts in a compound.

4. How many atoms in:

   • (i) An H₂S molecule: 3 (2 H + 1 S)
   • (ii) A PO₄³⁻ ion: 5 (1 P + 4 O)

Fun Questions

1. Scenario: If Sodium (Na) has a valency of 1 and Chlorine (Cl) has a valency of 1, what is the formula for Sodium Chloride?
   Answer: The formula is NaCl.

2. Scenario: Magnesium (Mg) with a valency of 2 combined with Chlorine (Cl) which has a valency of 1, what will the formula be?
   Answer: The formula is MgCl₂.

3. Scenario: If we have 2 Aluminum atoms (each Al with a valency of 3) and 6 Oxygen atoms (each O with a valency of 2), what is the compound?
   Answer: The formula is Al₂O₃.

4. Scenario: How many different types of atoms are in the formula Na₂CO₃?
   Answer: There are 3 types of atoms: Sodium (Na), Carbon (C), and Oxygen (O).

5. Scenario: If you have Calcium (Ca) and Nitrate (NO₃) together, what would it look like?
   Answer: The formula is Ca(NO₃)₂ for calcium nitrate.