Atoms, elements and compounds

What you need to know

• An atom is the smallest part of an element that can exist, and each element has a chemical symbol.
• There are about 100 elements in the periodic table; compounds contain two or more elements chemically combined in fixed proportions.
• Chemical reactions form new substances and are the only way compounds can be separated into their elements.
• Chemical language progresses from names and symbols to formulae, word equations and balanced equations; half equations and ionic equations are Higher Tier where appropriate.

The big picture

All substances are made of atoms, and each element is represented by its own chemical symbol. Compounds contain two or more elements chemically combined in fixed proportions and are formed by chemical reactions, which produce new substances and often involve a detectable energy change. Chemical formulae and equations use element symbols to represent substances and reactions. Students need to recognise and use element names and symbols, name compounds from formulae, and write word, formula and balanced chemical equations; Higher Tier also includes balanced half equations and ionic equations where appropriate.

From atoms to chemical equations

Follow the language of chemistry from the smallest idea to a complete reaction.

1. 1

   Start with atoms

   All substances are made of atoms, and an atom is the smallest part of an element that can exist.

2. 2

   Give elements symbols

   Each element is represented by a chemical symbol, allowing atoms and elements to be written compactly.

3. 3

   Combine elements chemically

   Chemical reactions form compounds from elements and always produce one or more new substances.

4. 4

   Write compound formulae

   Formulae use element symbols to represent compounds whose elements are chemically combined in fixed proportions.

5. 5

   Represent the reaction

   Reactions can be written as word equations or using formulae in balanced chemical equations.

6. 6

   Extend at Higher Tier

   Higher Tier students also write balanced half equations and ionic equations where appropriate.

The chemistry skills ladder

Different representations ask you to do different jobs.

Core language

These definitions hold the topic together.

Predict, then check

Decide what the information actually tells you before revealing the explanation.

You are shown a formula containing symbols for two different elements. What can you conclude if it represents a compound?

Memory hook

Atom → element symbol → compound formula → reaction equation: build chemical language one layer at a time.

Check yourself

What is the difference between an element symbol, a compound formula and a chemical equation?