Unit 2: Properties of materials

Compounds and formulae — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 7 parts Lesson 7 of 12 2 quizzes

📚 HIA International - Grade 8 Hub / Topical Revision

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Lesson summary

Compounds and formulae — HIA INTERNATIONAL notes: Explain the difference between elements and compounds, name compounds and use formulae.Compound = a substance in which two or more different kinds of atom are bonded together.Bonding — the chemical term for atoms joining tightly t…

Fixed proportion
New properties
Metal first
Ending -ide
Ending -ate
Number prefixes

Part 1 of 5What a compound is

Compound

Compound a substance in which two or more different kinds of atom are bonded together.

Bonding

Bonding the chemical term for atoms joining tightly together.

Fixed proportion

Fixed proportion a compound has a fixed ratio of atoms, e.g. water always has 2 hydrogen atoms to 1 oxygen atom.

New properties

New properties a compound has totally new properties, unlike the elements it is made from.

Example

Example sodium chloride (table salt) is not at all like sodium or chlorine.

Splitting

Splitting a compound can be split into its elements only by a chemical reaction.

Part 2 of 5Naming compounds

1Metal first

Metal first if the compound contains a metal, its name comes first.

2Ending -ide

Ending -ide the non-metal name changes, so sodium and chlorine make sodium chloride.

3Ending -ate

Ending -ate compounds with a third element, oxygen, such as calcium carbonate, calcium nitrate or lithium sulfate.

4Number prefixes

Number prefixes di- means two and mono- means one, e.g. carbon dioxide CO₂ and carbon monoxide CO.

5Everyday names

Everyday names sodium chloride is also called common salt.

Part 3 of 5Formulae

Formula

Formula the symbols of the elements bonded together in a compound, with small numbers showing how many atoms.

Subscript

Subscript the small number below and to the right of a symbol applies only to the atom before it.

No number

No number it means one atom of that element.

Brackets

Brackets they multiply everything inside, so Ca(OH)₂ has 1 Ca, 2 O and 2 H.

Name	Formula	Atoms
Calcium oxide	CaO	1 Ca, 1 O
Carbon dioxide	CO₂	1 C, 2 O
Hydrogen sulfide	H₂S	2 H, 1 S
Calcium carbonate	CaCO₃	1 Ca, 1 C, 3 O
Sodium hydroxide	NaOH	1 Na, 1 O, 1 H

Part 4 of 5Particle diagrams

Particle diagrams

Compound

Compound if different kinds of atom are bonded together in the particle.

Element

Element if all the atoms in the particle are the same, e.g. O₂.

Molecule

Molecule a particle of two or more atoms bonded together; it may be an element (O₂) or a compound (CO₂, H₂O, CH₄).

Hydroxides

Hydroxides compounds such as NaOH and KOH all contain oxygen and hydrogen.

Part 5 of 5Reading a formula

Identify each element symbol (a capital letter, sometimes followed by a small letter).

Read the subscript after each symbol; no number means 1.

Multiply by the number outside any bracket.

Add up the atoms of each element.

Name the compound with -ide for two elements or -ate when oxygen is also present.

Key terms

Term	Meaning
Compound	A substance whose particles contain two or more different kinds of atom bonded together.
Bonding	Atoms joining tightly together.
Formula	Symbols and numbers showing the atoms in a compound.
Molecule	A particle of two or more atoms bonded together.
Element	A substance made of one kind of atom.
Sodium chloride	The compound common salt, made of sodium and chlorine.

Avoid these mistakes

Compounds keep the properties of their elements — they have completely new properties.

CO and Co are the same — CO is carbon monoxide and Co is the element cobalt.

The subscript applies to the whole formula — it applies only to the symbol or bracket before it.

Names ending -ide contain oxygen — -ide means two elements; -ate includes oxygen.

Last updated: 12 October 2026