1. What defines an element?
- Neutron number alone
- Physical state
- Proton number
Check the answer and reasoning
Proton number
All atoms of one element have the same number of protons.
CHEM / Foundations
Atoms, elements and compounds
An element contains atoms with the same proton number. A compound contains two or more elements chemically combined in a fixed ratio.
Learning objectives
- Explain atoms, elements and compounds using a concrete example.
- Apply the worked method and justify each step.
- Recognise the limitation described in the final check.
The big idea
An element contains atoms with the same proton number. A compound contains two or more elements chemically combined in a fixed ratio.
Mixtures have variable composition and can often be separated by physical methods. Chemical symbols and formulae describe identities and ratios; a subscript belongs to the formula and is not a balancing coefficient.
Worked example
How many oxygen atoms are represented by 2CO₂?
Follow the solution
Each CO₂ contains two oxygen atoms and the coefficient counts two molecules: 2×2=4.
Investigate and explain
Predict the worked result before calculating. Change one input, record the new result, and explain which assumptions still hold.
Related interactive model. Compare its assumptions with the explanation; it is not a complete syllabus lesson.
Check your understanding
2. How many oxygen atoms are represented by 2CO₂?
- 4
- 2
- 3
Check the answer and reasoning
4
Each CO₂ contains two oxygen atoms and the coefficient counts two molecules: 2×2=4.
3. Can filtration separate dissolved salt from water?
- Yes
- No
- Only with cold water
Check the answer and reasoning
No
Dissolved ions pass through ordinary filter paper; evaporation or distillation is needed.