medium difficulty6 min

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.

In one line: Charges cross, signs drop, brackets protect radicals.

Practice This Concept

Understand it

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.

Rule to remember

Criss-cross rule: use ion charge numbers as subscripts, remove signs, and simplify the ratio when required.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Calcium nitrate has Ca²⁺ and NO₃^-.

1Criss-crossing gives Ca(NO₃)₂.
2Brackets are used because nitrate is a polyatomic ion repeated twice.

Method you can reuse

1Write cation and anion symbols.
2Write their charges.
3Criss-cross charge numbers as subscripts.
4Use brackets for repeated polyatomic ions.
5Simplify if both subscripts have a common factor.

Visual tip

A small charge-balance table can show cation, anion, charges, criss-cross subscripts, and final formula.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

How is the formula of magnesium chloride obtained from Mg²⁺ and Cl^-?

Think first, then show answer

Magnesium has charge 2+ and chloride has charge 1-. Criss-crossing the charge numbers gives MgCl₂, meaning one magnesium ion balances two chloride ions.

Common wrong answer

Writing CaNO₃₂.

Why it loses marks: Nitrate is a complete polyatomic ion; the correct formula is Ca(NO₃)₂.

Don't mix this up

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.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Asked as direct formula writing, correcting a wrong formula, or explaining why brackets are needed for polyatomic ions.

1
Enough for 1 mark
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.
2
Add for 2 marks
Example: For aluminium oxide, Al³⁺ and O²⁻ criss-cross to give Al₂O₃.
3
Add for 3 marks
For ionic compounds, the total positive and negative charges must balance.

MCQ Quiz

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