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Lesson 11: Chemical Formulae Formulae of elements and compounds Meaning of subscripts Simple formula writing
Year 8 • Chemistry
Lesson 11: Chemical Formulae
In this lesson, students learn how chemical formulae represent elements and compounds. The meaning of subscripts is explained clearly, and students practise writing simple chemical formulae.
Lesson Objectives
- Understand what chemical formulae show.
- Identify formulae of elements and compounds.
- Explain the meaning of subscripts.
- Write simple chemical formulae.
1. What Is a Chemical Formula?
A chemical formula shows which elements are present in a substance and how many atoms of each element there are.
[ Image Placeholder – Chemical Formula Examples ]
Examples:
• H₂O → water
• O₂ → oxygen
• CO₂ → carbon dioxide
• H₂O → water
• O₂ → oxygen
• CO₂ → carbon dioxide
2. Formulae of Elements
Some elements exist as single atoms, while others exist as molecules made of more than one atom.
[ Image Placeholder – Element Formulae ]
Examples:
• Helium → He
• Oxygen → O₂
• Nitrogen → N₂
• Helium → He
• Oxygen → O₂
• Nitrogen → N₂
3. Formulae of Compounds
A compound contains atoms of different elements chemically bonded together.
[ Image Placeholder – Compound Formulae ]
Examples:
• Water → H₂O
• Carbon dioxide → CO₂
• Sodium chloride → NaCl
• Water → H₂O
• Carbon dioxide → CO₂
• Sodium chloride → NaCl
4. Meaning of Subscripts
A subscript is the small number written after an element symbol.
[ Image Placeholder – Subscript Explanation ]
• H₂O → 2 hydrogen atoms, 1 oxygen atom
• CO₂ → 1 carbon atom, 2 oxygen atoms
• No number means only 1 atom
• CO₂ → 1 carbon atom, 2 oxygen atoms
• No number means only 1 atom
5. Writing Simple Chemical Formulae
Chemical formulae can be written by identifying the elements present and counting the atoms.
[ Image Placeholder – Writing Formulae Steps ]
Example:
Water contains hydrogen and oxygen → H₂O
Carbon dioxide contains carbon and oxygen → CO₂
Water contains hydrogen and oxygen → H₂O
Carbon dioxide contains carbon and oxygen → CO₂
Practice Questions
A. Fill in the Blanks
- A chemical __________ shows the elements present.
- The small number in a formula is called a __________.
- O₂ contains __________ oxygen atoms.
- CO₂ contains __________ oxygen atoms.
- No subscript means __________ atom.
B. True or False
- H₂O is an element.
- CO₂ is a compound.
- The subscript shows the number of atoms.
- NaCl has two different elements.
- O is the same as O₂.
C. Short Answer
- What does a chemical formula show?
- What does the subscript in H₂O mean?
- Give one example of an element formula.
- Give one example of a compound formula.
- How many atoms are in CO₂?
✅ Show Answer Key
A. Fill in the Blanks
- formula
- subscript
- two
- two
- one
B. True or False
- False
- True
- True
- True
- False
C. Short Answer
- The elements and number of atoms.
- There are two hydrogen atoms.
- O₂ / N₂ / He.
- H₂O / CO₂ / NaCl.
- Three atoms.
© Aviate Learning – Chemical Formulae (Year 8)
⚛️ Interactive Chemical Formula Builder
Build chemical formulae by choosing elements and subscripts.
This helps you understand how symbols and subscripts work.
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Build a formula to begin.