Introduction

In the previous lesson, you learned that atoms can gain or lose electrons to form ions. Positive ions (cations) and negative ions (anions) are attracted to each other because they carry opposite electrical charges.

This attraction forms one of the strongest types of chemical bonding, known as an ionic bond.

Ionic bonding is responsible for many familiar substances, including table salt (NaCl), magnesium oxide (MgO), and calcium chloride (CaCl₂).

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What is an Ionic Bond?

An ionic bond is the strong electrostatic attraction between oppositely charged ions.

The bond forms after one atom transfers one or more electrons to another atom.

This creates:

• A positive ion (cation)
• A negative ion (anion)

These oppositely charged ions attract one another, holding the compound together.

Unlike covalent bonding, electrons are transferred, not shared.

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How Does Ionic Bonding Form?

Ionic bonding usually occurs between:

• a metal
• a non-metal

The metal loses electrons.

The non-metal gains those electrons.

Example:

Sodium + Chlorine

Before:

• Sodium: 2,8,1
• Chlorine: 2,8,7

Electron transfer:

• Sodium loses one electron.
• Chlorine gains one electron.

After:

• Sodium ion: Na⁺
• Chloride ion: Cl⁻

The opposite charges attract, forming an ionic bond.

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Electrostatic Attraction

The force holding ionic compounds together is called electrostatic attraction.

Opposite charges attract just like opposite poles of magnets.

Positive ions attract negative ions in all directions.

This attraction is very strong.

It is responsible for the strength and stability of ionic compounds.

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Ionic Compounds

An ionic compound is a compound made from positive and negative ions.

Some common examples are:

Notice that every ionic compound contains:

• Positive ions
• Negative ions

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Why Are Ionic Compounds Neutral?

Although ions have charges, the overall compound has no net charge.

The total positive charge equals the total negative charge.

Examples:

Sodium chloride

Na⁺

Cl⁻

Total charge:

+1 + (−1) = 0

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Magnesium oxide

Mg²⁺

O²⁻

Total charge:

+2 + (−2) = 0

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Calcium chloride

Ca²⁺

2Cl⁻

Total charge:

+2 + (−1) + (−1) = 0

Every ionic compound must have a total charge of zero.

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Representing Ionic Bonding

Chemists often use dot-and-cross diagrams to show ionic bonding.

These diagrams show:

• Valence electrons
• Electron transfer
• The charges on the ions

Example:

Before transfer:

Na • Cl ×××××××

After transfer:

[Na]⁺ [Cl]⁻

The chloride ion now has a complete outer shell.

The sodium ion also has a full outer shell beneath it.

Dot-and-cross diagrams help us see exactly how ionic bonds form.

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Ionic Lattices

Individual ionic compounds do not exist as separate molecules.

Instead, millions of ions arrange themselves into a giant ionic lattice.

Each positive ion is surrounded by negative ions.

Each negative ion is surrounded by positive ions.

This arrangement:

• maximizes attraction
• minimizes repulsion
• makes ionic compounds very stable

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Recognizing Ionic Compounds from Their Formulas

Many ionic compounds can be recognized because they contain:

• A metal first
• A non-metal second

Examples:

A quick clue is that metal + non-metal usually indicates an ionic compound.

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Real-World Applications

Ionic compounds are used in many everyday products and industries.

Examples include:

• Table salt for food
• Road salt for melting ice
• Fertilizers
• Medicines
• Ceramics
• Glass production
• Electrolyte solutions
• Water treatment

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Worked Examples

Example 1

What type of bond forms between Na⁺ and Cl⁻?

Answer:

An ionic bond.

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Example 2

Why are Na⁺ and Cl⁻ attracted to one another?

Answer:

Because they have opposite electrical charges.

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Example 3

Is MgO electrically neutral?

Mg²⁺

O²⁻

Total charge:

+2 + (−2) = 0

Answer:

Yes.

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Example 4

Which compound is ionic?

A. CO₂

B. H₂O

C. NaCl

D. CH₄

Answer:

NaCl

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Example 5

Why is CaCl₂ written with two chlorine ions?

Answer:

One calcium ion has a charge of +2.

Each chloride ion has a charge of −1.

Two chloride ions are needed to balance the +2 charge and produce an overall neutral compound.

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Did You Know?

A single grain of table salt contains an enormous number of ions arranged in a repeating crystal lattice. Instead of existing as separate NaCl molecules, the sodium and chloride ions form one continuous three-dimensional structure that extends throughout the entire crystal.

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Key Terms

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Key Takeaways

• Ionic bonds form when electrons are transferred from a metal to a non-metal.
• Positive and negative ions are held together by strong electrostatic attraction.
• Ionic compounds always contain both cations and anions.
• The total positive and negative charges balance, making ionic compounds electrically neutral overall.
• Dot-and-cross diagrams help illustrate electron transfer and ion formation.
• Ionic compounds exist as giant crystal lattices rather than individual molecules.

Introduction

Chemists use chemical formulas to show which elements are present in a compound and the ratio in which their atoms or ions combine.

For ionic compounds, the formula is not chosen randomly. It is determined by one simple but important rule:

The total positive charge must equal the total negative charge.

This rule, known as charge balance, allows us to predict the correct formula for almost any ionic compound.

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Why Do Ionic Compounds Need Balanced Charges?

Every ionic compound must be electrically neutral.

This means:

Total positive charge = Total negative charge

For example:

Na⁺ + Cl⁻

Total charge:

+1 + (−1) = 0

Formula:

NaCl

Only one sodium ion and one chloride ion are needed.

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Finding the Correct Ion Ratio

Sometimes one positive ion is not enough to balance one negative ion.

Instead, we need the correct ratio of ions.

Example 1

Magnesium forms:

Mg²⁺

Chlorine forms:

Cl⁻

One Mg²⁺ has a charge of +2.

Each Cl⁻ has a charge of −1.

Two chloride ions are needed.

Formula:

MgCl₂

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Example 2

Calcium oxide

Calcium:

Ca²⁺

Oxygen:

O²⁻

Charges:

+2 and −2

One of each balances perfectly.

Formula:

CaO

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Example 3

Aluminium oxide

Aluminium:

Al³⁺

Oxygen:

O²⁻

The smallest combination that balances the charges is:

2 Al³⁺ = +6

3 O²⁻ = −6

Formula:

Al₂O₃

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Using Charge Balance

A simple method for writing ionic formulas is:

Step 1

Write the ion symbols and charges.

Example:

Ca²⁺

Cl⁻

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Step 2

Determine how many of each ion are needed so the total charge equals zero.

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Step 3

Write the chemical formula.

Result:

CaCl₂

Remember:

The charges are not written in the final formula.

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Common Ionic Charges

The periodic table helps predict the charges of many common ions.

Some common ions are:

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Naming Ionic Compounds

Naming ionic compounds is straightforward.

Rule

1. Name the positive ion first.
2. Name the negative ion second.
3. Change the ending of the non-metal to -ide.

Examples:

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Identifying the Ions

You should also be able to determine which ions make up a compound.

Example:

NaCl

Contains:

• Na⁺
• Cl⁻

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Example:

MgCl₂

Contains:

• Mg²⁺
• 2 Cl⁻

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Example:

Al₂O₃

Contains:

• 2 Al³⁺
• 3 O²⁻

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The Criss-Cross Method

A useful shortcut is the criss-cross method.

Take the charge numbers (not the signs) and use them as subscripts.

Example:

Al³⁺

O²⁻

Cross the numbers:

Al₂O₃

However, always simplify the ratio if possible.

Example:

Ca²⁺

O²⁻

Criss-cross gives:

Ca₂O₂

Simplify:

CaO

The formula should always show the smallest whole-number ratio of ions.

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Real-World Applications

Writing ionic formulas is important in:

• Medicine
• Agriculture
• Chemical manufacturing
• Water treatment
• Food production
• Materials science
• Environmental science
• Laboratory chemistry

Chemists use formulas to communicate exactly which substances are involved in chemical reactions.

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Worked Examples

Example 1

Write the formula for sodium chloride.

Na⁺

Cl⁻

Charges balance 1:1.

Answer:

NaCl

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Example 2

Write the formula for magnesium oxide.

Mg²⁺

O²⁻

Charges balance 1:1.

Answer:

MgO

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Example 3

Write the formula for calcium chloride.

Ca²⁺

Cl⁻

Two chloride ions are needed.

Answer:

CaCl₂

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Example 4

Write the formula for aluminium oxide.

Al³⁺

O²⁻

Balance:

2 Al³⁺

3 O²⁻

Answer:

Al₂O₃

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Example 5

Name the compound:

K₂S

Contains:

K⁺

S²⁻

Answer:

Potassium sulfide

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Example 6

Identify the ions in MgBr₂.

Answer:

• Mg²⁺
• 2 Br⁻

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Did You Know?

The chemical formulas of ionic compounds always represent the simplest ratio of ions, not the actual number of ions in a crystal. A tiny crystal of sodium chloride contains billions of sodium and chloride ions arranged in a giant repeating lattice, yet its formula is simply NaCl because the ratio of sodium ions to chloride ions is always 1:1.

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Key Terms

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Key Takeaways

• Ionic formulas are determined by balancing positive and negative charges.
• Every ionic compound must have an overall charge of zero.
• Subscripts show the ratio of ions, not their charges.
• The formula always uses the smallest whole-number ratio of ions.
• Ionic compounds are named by writing the positive ion first and the negative ion second, with the non-metal ending changed to -ide.
• Understanding charge balance allows you to write and interpret formulas for a wide range of ionic compounds.

Introduction

When people think about table salt (NaCl), they often imagine tiny particles called "salt molecules." However, this is not how ionic compounds exist.

Instead of forming separate molecules, ionic compounds form giant three-dimensional structures called crystal lattices. These lattices contain millions—or even billions—of positive and negative ions arranged in a regular repeating pattern.

The crystal lattice is responsible for many of the unique properties of ionic compounds, including their hardness, high melting points, and ability to conduct electricity when molten or dissolved in water.

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What is a Crystal Lattice?

A crystal lattice is a regular, repeating three-dimensional arrangement of ions.

The lattice extends in every direction, creating one giant structure.

Instead of existing as separate NaCl molecules, sodium chloride forms a continuous network of alternating sodium and chloride ions.

Each ion occupies a fixed position within the lattice.

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Why Do Ionic Compounds Form Lattices?

Every positive ion attracts every nearby negative ion.

Likewise, every negative ion attracts every nearby positive ion.

The ions arrange themselves to:

• maximize attractive forces
• minimize repulsive forces
• produce the most stable arrangement possible

This repeating arrangement is the crystal lattice.

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The Arrangement of Ions

In an ionic lattice:

• Positive ions are surrounded by negative ions.
• Negative ions are surrounded by positive ions.

For example, in sodium chloride:

• Every Na⁺ ion is surrounded by several Cl⁻ ions.
• Every Cl⁻ ion is surrounded by several Na⁺ ions.

This arrangement creates strong attractions throughout the entire crystal.

No ion is bonded to just one neighboring ion—it is attracted to many ions around it.

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Giant Ionic Structures

A crystal lattice is called a giant ionic structure because it contains an enormous number of ions arranged in a repeating pattern.

Unlike covalent substances such as water or carbon dioxide, ionic compounds do not consist of individual molecules.

Instead, the whole crystal acts as one continuous structure.

Examples include:

• Sodium chloride (NaCl)
• Magnesium oxide (MgO)
• Calcium fluoride (CaF₂)
• Potassium bromide (KBr)

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Crystal Lattices vs Molecules

One common misconception is that NaCl is a molecule.

It is not.

Compare the two types of substances:

Understanding this difference is very important in chemistry.

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How the Lattice Explains Ionic Bonding

The strong electrostatic attraction between oppositely charged ions acts in all directions throughout the lattice.

This means each ion is attracted to many neighboring ions at the same time.

These many attractions make ionic compounds:

• very stable
• difficult to separate
• strong solids at room temperature

The lattice exists because ionic bonding occurs throughout the entire crystal, not just between pairs of ions.

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Interpreting Ionic Structure Diagrams

Chemists often use simplified diagrams to represent crystal lattices.

The diagrams usually show:

• positive ions
• negative ions
• alternating arrangement
• repeating pattern

Remember that these diagrams show only a small portion of the complete lattice.

The actual crystal continues in all directions.

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Why Are Ionic Crystals Hard?

When force is applied, many ions resist being pulled apart because of the strong electrostatic attractions holding the lattice together.

This is why ionic compounds are generally:

• hard
• rigid
• difficult to deform

However, they are also brittle.

If layers shift, ions with the same charge may be forced next to each other.

Since like charges repel, the crystal can crack or shatter.

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Real-World Applications

Crystal lattices are important in many materials used every day.

Examples include:

• Table salt
• Ceramic materials
• Minerals
• Building materials
• Glass manufacturing
• Batteries
• Electronic components
• Industrial chemicals

Scientists study crystal lattices to develop stronger materials, better batteries, and new electronic devices.

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Worked Examples

Example 1

What is a crystal lattice?

Answer:

A giant repeating three-dimensional arrangement of positive and negative ions.

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Example 2

Why do ionic compounds form crystal lattices?

Answer:

Because opposite charges attract in all directions, producing the most stable arrangement of ions.

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Example 3

Does sodium chloride exist as individual NaCl molecules?

Answer:

No.

It exists as a giant ionic lattice.

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Example 4

What surrounds each sodium ion in a sodium chloride crystal?

Answer:

Several chloride ions.

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Example 5

Why are ionic crystals usually hard?

Answer:

Because many strong electrostatic attractions hold the ions firmly in place throughout the crystal lattice.

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Did You Know?

Natural table salt forms beautiful cubic crystals because of the way sodium and chloride ions arrange themselves in a repeating cubic lattice. This regular arrangement at the atomic level determines the crystal's external shape, which is why salt crystals often look like tiny cubes under a microscope.

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Key Terms

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Key Takeaways

• Ionic compounds form giant crystal lattices rather than individual molecules.
• Crystal lattices are repeating three-dimensional arrangements of positive and negative ions.
• Strong electrostatic attractions act throughout the entire lattice.
• The lattice structure makes ionic compounds stable, hard, and generally high-melting.
• Diagrams of ionic structures represent only a small part of a much larger repeating lattice.
• Understanding crystal lattices helps explain the properties of ionic compounds and their importance in many natural and manufactured materials.

Introduction

The properties of a substance depend on how its particles are arranged and held together.

In the previous lesson, you learned that ionic compounds consist of giant crystal lattices made of positive and negative ions held together by strong electrostatic attractions.

These strong attractions give ionic compounds many distinctive properties, including:

• High melting and boiling points
• Hard but brittle structures
• Electrical conductivity when molten or dissolved in water
• Crystalline solids at room temperature

By understanding the structure of ionic compounds, we can explain why they behave the way they do.

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Property 1: High Melting and Boiling Points

Ionic compounds usually have very high melting and boiling points.

For example:

Why?

The positive and negative ions are held together by strong electrostatic attractions.

A large amount of energy is required to overcome these attractions.

The stronger the attractions, the higher the melting and boiling points.

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Property 2: Hard Solids

Ionic compounds are generally hard solids.

The ions are locked into fixed positions within the crystal lattice.

Since the ions cannot move easily, the solid resists being scratched or compressed.

Examples include:

• Table salt
• Magnesium oxide
• Calcium fluoride

These compounds maintain their shape because the ions are strongly held together.

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Property 3: Brittleness

Although ionic compounds are hard, they are also brittle.

If enough force is applied, layers of ions can shift.

When this happens:

• Positive ions may move beside other positive ions.
• Negative ions may move beside other negative ions.

Since like charges repel, the crystal cracks or shatters.

This explains why ionic crystals break rather than bend.

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Property 4: Electrical Conductivity

Electrical current is the movement of charged particles.

Since ions are charged, you might expect ionic compounds to conduct electricity.

However, this depends on whether the ions can move.

Ionic solids

In a solid crystal:

• ions are fixed in position
• they cannot move

Result:

Do not conduct electricity

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Molten ionic compounds

When melted:

• ions become free to move

Result:

Conduct electricity

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Ionic compounds dissolved in water

When dissolved:

• ions separate from one another
• ions move freely through the solution

Result:

Conduct electricity

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Why Do Metals Conduct Differently?

It is useful to compare ionic compounds with metals.

The particles responsible for conduction are different.

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Property 5: Crystalline Appearance

Many ionic compounds form crystals with:

• flat surfaces
• straight edges
• regular shapes

This happens because the ions are arranged in an orderly repeating lattice.

Examples include:

• Table salt
• Potassium bromide
• Calcium fluoride

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Relating Properties to Structure

Every property of an ionic compound can be explained by its structure.

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Predicting Ionic Properties

If you know a substance has ionic bonding, you can predict that it is likely to:

• have a high melting point
• have a high boiling point
• be hard
• be brittle
• conduct electricity when molten
• conduct electricity when dissolved in water
• not conduct electricity as a solid
• form a crystal lattice

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Real-World Applications

The properties of ionic compounds make them useful in many applications.

Examples include:

• Road salt for melting ice
• Electrolysis to produce metals and chemicals
• Ceramic materials
• Electrolyte solutions in medicine
• Batteries
• Water treatment
• Fertilizers

Scientists select ionic compounds because their properties suit particular uses.

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Worked Examples

Example 1

Why does sodium chloride have a high melting point?

Answer:

Because strong electrostatic attractions between ions require a large amount of energy to overcome.

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Example 2

Why does solid sodium chloride not conduct electricity?

Answer:

Its ions are fixed in the crystal lattice and cannot move.

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Example 3

Why does molten sodium chloride conduct electricity?

Answer:

The ions are free to move and carry electric charge.

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Example 4

Why are ionic compounds brittle?

Answer:

When layers shift, like charges are brought together. The repulsion between like charges causes the crystal to crack or shatter.

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Example 5

Predict two properties of magnesium oxide.

Answer:

Possible answers include:

• High melting point
• Hard solid
• Brittle
• Conducts electricity when molten
• Does not conduct electricity as a solid

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Did You Know?

Molten ionic compounds are used in a process called electrolysis to produce important substances such as aluminium, chlorine, and sodium. During electrolysis, the moving ions carry electric current through the molten liquid, allowing chemical reactions to occur that would not be possible in the solid state.

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Key Terms

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Key Takeaways

• Ionic compounds have high melting and boiling points because strong electrostatic attractions hold the ions together.
• They are hard because the ions are locked into fixed positions within a crystal lattice.
• They are brittle because shifting the lattice brings like charges together, causing repulsion.
• Solid ionic compounds do not conduct electricity because the ions cannot move.
• Molten and aqueous ionic compounds conduct electricity because the ions are free to move.
• The physical properties of ionic compounds can be explained by their giant crystal lattice structure and ionic bonding.