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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.

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Student-friendly 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.

How to write this in exams

  1. 1

    Start with the exact idea

    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. 2

    Then show how to use it

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

  3. 3

    Add one concrete example

    For aluminium oxide, Al3+ and O2- criss-cross to give Al2O3.

  4. 4

    Avoid this incomplete answer

    Writing CaNO32 is wrong because nitrate is a complete polyatomic ion; the correct formula is Ca(NO3)2.

Definition

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.

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.

Memory hook

Charges cross, signs drop, brackets protect radicals.

Examples and method

Worked example

Calcium nitrate has Ca2+ and NO3-. Criss-crossing gives Ca(NO3)2. Brackets are used because nitrate is a polyatomic ion repeated twice.

Method to apply

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

Diagram support

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

How CBSE asks it

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

Avoid common mistakes

Common confusion

Students often write charges inside the final formula, such as Al2+O3-, instead of writing only symbols and subscripts.

Common wrong answer

Writing CaNO32 is wrong because nitrate is a complete polyatomic ion; the correct formula is Ca(NO3)2.

Exam tip

Write ions with charges first, criss-cross charge numbers, remove charge signs, and reduce the ratio if needed.

Quick check

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

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

Answer writing and exam use

1-mark answer

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-mark answer

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. Criss-cross rule: use ion charge numbers as subscripts, remove signs, and simplify the ratio when required. For aluminium oxide, Al3+ and O2- criss-cross to give Al2O3.

3-mark answer

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. Criss-cross rule: use ion charge numbers as subscripts, remove signs, and simplify the ratio when required. Calcium nitrate has Ca2+ and NO3-. Criss-crossing gives Ca(NO3)2. Brackets are used because nitrate is a polyatomic ion repeated twice. Asked as direct formula writing, correcting a wrong formula, or explaining why brackets are needed for polyatomic ions. Writing CaNO32 is wrong because nitrate is a complete polyatomic ion; the correct formula is Ca(NO3)2.
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