Writing Chemical Formulae
Writing chemical formulae means representing a compound using symbols of elements or radicals and the correct number of each particle needed to balance valencies or charges.
Practice This ConceptMain explanation
Teacher explanation
For ionic compounds, the total positive and negative charges must balance. The criss-cross method helps write formulae by using the magnitude of charges as subscripts, then simplifying if possible.
Example
For aluminium oxide, Al3+ and O2- criss-cross to give Al2O3.
Simple analogy
Charges cross, signs drop, brackets protect radicals.
Common confusion
Students often write charges inside the final formula, such as Al2+O3-, instead of writing only symbols and subscripts.
Exam tip
Write ions with charges first, criss-cross charge numbers, remove charge signs, and reduce the ratio if needed.
Answer writing and exam use
1-mark use
Write the exact meaning of writing chemical formulae in one clean line.
2-mark use
Define writing chemical formulae and add one example or condition.
3-mark use
Explain writing chemical formulae, show the method or example, and mention the common mistake.
Practice this concept with focused MCQs
Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.
Help improve this page
Found something confusing, incorrect, or missing?