Science with Tara

In this lesson, we will learn about common ions and how to write the chemical formulas of ionic compounds.

By the end of this article, you will be able to:

• Recall the charges of common positive and negative ions
• Identify polyatomic ions
• Write the chemical formula of ionic compounds correctly

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1. Formation of Positive Ions (Cations)

Positive ions are called cations. They are formed when atoms lose electrons.

In general, metals tend to lose electrons and form cations. The charge of the ion is related to the group number of the element in the Periodic Table.

Group in Periodic Table	Element	Ion	Formula	Charge
1	Sodium	Sodium ion	Na⁺	+1
1	Potassium	Potassium ion	K⁺	+1
2	Magnesium	Magnesium ion	Mg²⁺	+2
2	Calcium	Calcium ion	Ca²⁺	+2
13	Aluminium	Aluminium ion	Al³⁺	+3
Transition	Iron(II)	Iron(II) ion	Fe²⁺	+2
Transition	Iron(III)	Iron(III) ion	Fe³⁺	+3
Transition	Copper(I)	Copper(I) ion	Cu⁺	+1
Transition	Copper(II)	Copper(II) ion	Cu²⁺	+2
Transition	Silver	Silver ion	Ag⁺	+1
Transition	Zinc	Zinc ion	Zn²⁺	+2
—	Hydrogen	Hydrogen ion	H⁺	+1

Note: Hydrogen is a non-metal but can form a positive ion (H⁺).

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2. Formation of Negative Ions (Anions)

Negative ions are called anions. They are formed when atoms gain electrons.

In general, non-metals tend to gain electrons and form anions.

Group in Periodic Table	Element	Ion Name	Formula	Charge
17	Fluorine	Fluoride	F⁻	–1
17	Chlorine	Chloride	Cl⁻	–1
17	Bromine	Bromide	Br⁻	–1
17	Iodine	Iodide	I⁻	–1
16	Oxygen	Oxide	O²⁻	–2
15	Nitrogen	Nitride	N³⁻	–3

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3. Polyatomic Ions

A polyatomic ion is an ion made up of two or more atoms joined together that carries an overall charge.

These are the common polyatomic ions you need to remember:

Name	Formula
Ammonium	NH₄⁺
Hydroxide	OH⁻
Nitrate	NO₃⁻
Sulfate	SO₄²⁻
Carbonate	CO₃²⁻
Sulfite	SO₃²⁻

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4. How to Write Chemical Formulas of Ionic Compounds

Follow these steps:

1. Write the symbol of the positive ion and the negative ion.
2. Write their charges.
3. Balance the charges so that the overall compound is neutral (use the cross method if needed).
4. Write the final formula (do not include the charges).

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Worked Examples

Example 1: Sodium chloride

• Sodium forms Na⁺
• Chlorine forms Cl⁻
• Charges are +1 and –1 → they balance Formula = NaCl

Example 2: Magnesium oxide

• Magnesium forms Mg²⁺
• Oxygen forms O²⁻
• Charges are equal → ratio 2:2, simplest ratio 1:1 Formula = MgO

Example 3: Aluminium oxide

• Aluminium forms Al³⁺
• Oxygen forms O²⁻
• Using cross method → Al₂O₃ Formula = Al₂O₃

Example 4: Calcium hydroxide

• Calcium forms Ca²⁺
• Hydroxide is OH⁻
• Ratio = 1:2 Formula = Ca(OH)₂ (Note: Brackets are needed for polyatomic ions)

Example 5: Iron(III) sulfate

• Iron(III) is Fe³⁺
• Sulfate is SO₄²⁻
• Ratio = 2:3 Formula = Fe₂(SO₄)₃

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Practice Questions

Try writing the formulas for the following compounds:

1. Sodium nitrate
2. Calcium carbonate
3. Aluminium chloride
4. Copper(II) oxide
5. Ammonium sulfate

Watch the tutorial to learn how to write the chemical formula of each ionic compound.

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Summary

• Metals form positive ions (cations).
• Non-metals form negative ions (anions).
• Always balance the charges when writing the formula of an ionic compound.
• Use brackets when there is more than one polyatomic ion.

Previous lesson: Four types of Bond Structures
Next lesson: Hydrogen Dot and Cross Diagram
All topics: Chemistry Resources