S4.7.1 For the adult
Chemical formulae
Why is salt written NaCl, but magnesium chloride written MgCl₂?
What this chapter teaches
A formula says which elements a substance has and how many atoms or ions of each. For an ionic compound, the numbers are the smallest that make the charges add to zero, and a bracket groups a polyatomic ion that is needed more than once.
Three questions to ask
There is no one right wording. Listen for the idea.
- Why does magnesium chloride need two chloride ions for each magnesium ion?
- How many atoms of each element are in Ca(OH)₂, and where did you use the bracket?
- What is the difference between a small low number and a small raised sign in a formula?
After the chapter, the learner can
- Write formulae for common compounds using ion charges
- Count the atoms of each element in a formula
What they copy into their notebook
Write the positive ion first. Use the smallest numbers that make the charges add to zero. Write them low, after the symbol. Put a bracket round a group that is counted more than once.