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Chemical Formula Writing Practice

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Chemical Formula Writing Practice

Write neutral ionic compound formulas by balancing positive and negative charges. Use the ion reference grid when needed. Reduce subscripts to the smallest whole-number ratio, and use brackets around a polyatomic ion when more than one is needed.

Positive ions Na⁺ sodium Mg²⁺ magnesium Al³⁺ aluminium Fe²⁺ iron(II) Fe³⁺ iron(III) NH₄⁺ ammonium

Negative ions Cl⁻ chloride O²⁻ oxide NO₃⁻ nitrate SO₄²⁻ sulfate CO₃²⁻ carbonate HCO₃⁻ hydrogen carbonate

Remember The total positive charge and total negative charge must add to zero. Use brackets if you need more than one polyatomic ion.

Balance the charges

Write each compound as a neutral formula. Show your thinking by stating how many of each ion are needed.

1.A lab assistant combines sodium ions (Na⁺) and chloride ions (Cl⁻) to make a neutral compound. Write the formula and state the ion ratio.

2.A student writes Mg₂O₂ for magnesium oxide because magnesium is Mg²⁺ and oxide is O²⁻. Which formula-writing rule corrects this?

Keep Mg₂O₂ because every charge becomes a subscript.
Reduce the ratio to the smallest whole numbers, giving MgO.
Change the formula to MgO₂ so oxygen has a subscript.
Write the charges as superscripts in the final formula.

3.Aluminium ions are Al³⁺ and chloride ions are Cl⁻. Work out how many of each ion are needed for a neutral compound, then write its formula.

4.Choose the correct description of a subscript in a chemical formula such as AlCl₃.

It shows the charge on the ion.
It shows the number of atoms or ions of the preceding species.
It shows the number of compounds in a sample.
It shows that the compound has an overall charge.

Polyatomic ions and tricky charges

A polyatomic ion is a group of bonded atoms that carries an overall charge and acts as a single ion in an ionic formula.

5.Calcium ions (Ca²⁺) combine with nitrate ions (NO₃⁻). Write the neutral formula, including brackets if they are needed.

6.A fertiliser label lists ammonium sulfate. Use NH₄⁺ and SO₄²⁻ to write its formula.

7.Which statement best explains why brackets appear in Ca(NO₃)₂?

Brackets show that calcium has a charge of 2−.
Brackets group the nitrate ion, and the outside subscript 2 applies to the whole group.
Brackets mean the nitrate ion is covalent and not charged.
Brackets are used whenever a formula contains oxygen.

8.Write the formula for aluminium sulfate using Al³⁺ and SO₄²⁻. Explain the smallest ion ratio that makes the total charge zero.

9.Iron can form Fe²⁺ or Fe³⁺. Write the formula for iron(II) chloride and iron(III) chloride, using Cl⁻ for chloride.

10.A classmate says that NaHCO₃ must be covalent because it contains several different elements. Explain why this reasoning is incorrect, and identify the ions in sodium hydrogen carbonate.