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Year 8

Chemical formulae

I can use chemical symbols to write down chemical formulae for elements and compounds.

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New
New
Year 8

Chemical formulae

I can use chemical symbols to write down chemical formulae for elements and compounds.

Download all resources
Share activities with pupils

Lesson details

Key learning points

  1. The symbols for each element (each type of atom) a substance contains, appear in its chemical formula.
  2. Numbers (subscripts) next to chemical symbols in a formula show the numbers of each type of atom in a compound.
  3. Each compound contains a different combination of elements.
  4. Some compounds contain the same elements as other compounds, but in a different ratio.

Common misconception

Writing subscript values as superscript values.

Subscript values show the number of the atoms, while superscript in maths is used to square a value and so does not apply to chemical formulae.

Keywords

  • Chemical symbol - One or two letters used to represent a chemical element. The first letter is always a capital letter, e.g. Br represents bromine.

  • Chemical formula - A representation of either the number of atoms or ratio of atoms of each element in a substance, e.g. H₂O is water = 2 x hydrogen + 1 x oxygen

Use many examples to show that superscript values are not correct, i.e. the element cannot be squared. Also model many examples of formulae, breaking down each element and how many atoms of each are present.
Teacher tip

Equipment

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.
Which element does the chemical symbol 'O' represent?
Correct Answer: oxygen, Oxygen
Q2.
In the chemical formula CO₂, what element does 'C' represent?
Correct Answer: carbon, Carbon
Q3.
Which of the following is a molecule?
Na
Correct answer: H₂O
Mg
Correct answer: NH₃
Q4.
Match the chemical formula to the correct substance.
Correct Answer:oxygen atom,O

O

Correct Answer:elemental oxygen,O₂

O₂

Correct Answer:molecule made of hydrogen and oxygen,H₂O

H₂O

Correct Answer:molecule of hydrogen,H₂

H₂

Q5.
Each is represented by its own chemical symbol, for example 'K', 'Fe' or 'C'.
Correct Answer: element, Element
Q6.
Match each term with its correct definition.
Correct Answer:element,A substance that cannot be broken down into simpler substances.

A substance that cannot be broken down into simpler substances.

Correct Answer:atom,The smallest unit of a chemical element.

The smallest unit of a chemical element.

Correct Answer:matter,Anything that has mass and takes up space.

Anything that has mass and takes up space.

Correct Answer:pure,A substance composed of only one chemical substance.

A substance composed of only one chemical substance.

Correct Answer:molecule,Two or more atoms bonded together (usually non-metals).

Two or more atoms bonded together (usually non-metals).

Correct Answer:compound,Two or more different types of atoms bonded together.

Two or more different types of atoms bonded together.

Exit quiz

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6 Questions

Q1.
What does the chemical formula H₂O represent?
Correct answer: Two hydrogen atoms and one oxygen atom.
One hydrogen atom and two oxygen atoms.
Two hydrogen molecules and one oxygen molecule.
Two oxygen atoms and one hydrogen atom.
Q2.
In the formula CO₂, what does the subscript '2' indicate?
There are two carbon atoms.
Correct answer: There are two oxygen atoms.
The compound is twice as heavy as oxygen.
The compound is twice as reactive.
Q3.
It is important to write subscript values correctly, both in terms of the value, but also in their physical size and placement to accurately represent the correct number of each type of ...
Correct Answer: atom, atom.
Q4.
What is the difference between H₂O and H₂O₂?
H₂O₂ has more hydrogen atoms.
H₂O has more oxygen atoms.
Correct answer: H₂O₂ has more oxygen atoms.
H₂O has more hydrogen atoms.
There's no difference. They have the same number of oxygen and hydrogen atoms.
Q5.
Which statement best defines a chemical formula?
A description of a chemical reaction.
A graphical representation of a molecule.
Correct answer: A symbolic way to represent the elements in a chemical substance.
A method to determine the pH of a substance.
Q6.
Match the following chemical substances up to the key terms.
Correct Answer:Element,Ne

Ne

Correct Answer:Compound,CO

CO

Correct Answer:Molecule,O₂

O₂

Correct Answer:Mixture,N₂ and CO₂

N₂ and CO₂