Atoms, Elements and Compounds
Revise Atoms, Elements and Compounds for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. An element contains only one type of atom, while a compound has two or more elements chemically bonded together in fixed proportions.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
All substances are made of atoms, and an atom is the smallest part of an element that can exist. There are about 100 different elements, shown in the periodic table.
An element contains only one type of atom. A compound contains two or more different elements chemically combined in fixed proportions, and can only be separated back into elements by chemical reactions. A mixture contains two or more substances not chemically combined.
That distinction matters because it determines how a substance can be separated. Compounds need a chemical reaction such as electrolysis, whereas mixtures can be separated by physical means such as filtration or distillation, because the substances retain their own properties.
Revision notes
Atoms and elements
An atom is the smallest part of an element that can exist. Elements are made of only one type of atom.
There are about 100 different elements, shown in the periodic table. Each is represented by a symbol of one or two letters, such as O for oxygen and Na for sodium.
Compounds
A compound contains two or more elements chemically combined in fixed proportions.
Compounds can be represented by formulae, such as H₂O and CO₂. They can only be separated into elements by chemical reactions, and their properties differ from those of the elements they contain.
Mixtures
A mixture consists of two or more substances not chemically combined together.
The chemical properties of each substance are unchanged, so mixtures can be separated by physical processes such as filtration, crystallisation, distillation and chromatography.
Key points
- An atom is the smallest part of an element that can exist.
- An element contains only one type of atom.
- A compound has elements chemically combined in fixed proportions.
- Compounds are separated only by chemical reactions.
- A mixture is not chemically combined.
- Mixtures are separated by physical processes.
Worked examples
Example 1
State the difference between a compound and a mixture. [2 marks]
Model answer
In a compound the elements are chemically combined in fixed proportions (1 mark), whereas in a mixture the substances are not chemically combined and each keeps its own properties (1 mark).
Example 2
Explain why a mixture can be separated by physical means but a compound cannot. [2 marks]
Model answer
In a mixture the substances are not chemically bonded, so they retain their own properties and can be separated physically (1 mark). In a compound the elements are held together by chemical bonds, which can only be broken by a chemical reaction (1 mark).
Example 3
Water has the formula H₂O. State what this tells you about the compound. [2 marks]
Model answer
It contains the elements hydrogen and oxygen chemically combined (1 mark), in the fixed proportion of two hydrogen atoms to every one oxygen atom (1 mark).
Common mistakes
Saying a compound is a mixture of elements.
In a compound the elements are chemically combined; in a mixture they are not.
Thinking a compound has the same properties as its elements.
Water is a liquid, but hydrogen and oxygen are both gases.
Saying mixtures need chemical reactions to separate.
Physical processes such as filtration and distillation are sufficient.
Writing element symbols with two capitals.
Only the first letter is capitalised — Na, not NA.
Exam tips
- Use the phrase chemically combined when defining a compound.
- Say fixed proportions to distinguish compounds from mixtures.
- Link the separation method to whether bonds must be broken.
- Write element symbols with one capital letter only.
Key terms
- Atom
- The smallest part of an element that can exist.
- Element
- A substance made of only one type of atom.
- Compound
- Two or more elements chemically combined in fixed proportions.
- Mixture
- Two or more substances not chemically combined.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.