Introduction

Nearly every substance on Earth is held together by chemical bonds. Two of the most important types of chemical bonding are ionic bonding and covalent bonding. Although both types of bonding allow atoms to achieve stable electron arrangements, they occur in very different ways and produce substances with very different properties.

By understanding how ionic and covalent compounds form, we can explain why table salt dissolves in water and conducts electricity when dissolved, while sugar dissolves but does not conduct electricity. We can also predict the behaviour of unfamiliar compounds simply by examining the elements they contain.

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Two Types of Chemical Bonding

The two main types of chemical bonding are:

• Ionic bonding
• Covalent bonding

Both produce stable compounds, but they use different methods to achieve full outer electron shells.

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Figure 1. Ionic bonding involves electron transfer, while covalent bonding involves electron sharing.

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How Ionic Bonds Form

Ionic bonding occurs when:

• A metal loses electrons.
• A non-metal gains those electrons.

This forms:

• Positive ions (cations)
• Negative ions (anions)

The oppositely charged ions attract one another through electrostatic forces.

Example:

Na → Na⁺ + e⁻

Cl + e⁻ → Cl⁻

Na⁺ + Cl⁻ → NaCl

Ionic compounds form giant three-dimensional ionic lattices.

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How Covalent Bonds Form

Covalent bonding occurs when:

• Two non-metals share electrons.

Instead of transferring electrons, atoms share one or more pairs of electrons.

Examples include:

• H₂O
• CO₂
• CH₄
• NH₃

Most covalent substances consist of individual molecules.

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Figure 2. Ionic compounds form giant lattices, while covalent compounds usually exist as individual molecules.

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Identifying Ionic and Covalent Compounds

A simple way to predict the type of bonding is to examine the elements involved.

Ionic Compounds

Usually contain:

• Metal + Non-metal

Examples:

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Covalent Compounds

Usually contain:

• Non-metal + Non-metal

Examples:

This rule correctly identifies most simple compounds.

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Comparing Bonding Mechanisms

Although both produce stable compounds, the structures they form are very different.

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Figure 3. Ionic compounds form through electron transfer, while covalent compounds form through electron sharing.

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Comparing Physical Properties

Because their structures are different, ionic and covalent compounds have different physical properties.

These differences arise from the different types of bonding.

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Why Ionic Compounds Have High Melting Points

In an ionic lattice:

• Every positive ion is attracted to surrounding negative ions.
• These electrostatic attractions are very strong.

A large amount of energy is required to separate the ions.

As a result:

• High melting point.
• High boiling point.

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Why Covalent Compounds Usually Have Lower Melting Points

Most covalent substances consist of small molecules.

Within each molecule:

• Covalent bonds are very strong.

However:

The forces between neighbouring molecules are much weaker.

Only these weaker intermolecular forces need to be overcome during melting.

Therefore, many covalent substances have relatively low melting and boiling points.

Examples:

• Oxygen
• Carbon dioxide
• Methane

Some giant covalent structures, such as diamond and silicon dioxide, are important exceptions because they have very strong covalent bonds throughout the entire structure.

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Figure 4. The structures of ionic and covalent substances explain their different melting and boiling points.

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Electrical Conductivity

Ionic Compounds

Solid ionic compounds:

• Do not conduct electricity.

Molten or dissolved ionic compounds:

• Conduct electricity.

Reason:

The ions become free to move and carry electric current.

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Covalent Compounds

Most covalent substances:

• Do not conduct electricity.

Reason:

They contain:

• No free ions.
• No mobile electrons.

Exception:

Graphite conducts electricity because it contains mobile electrons.

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Solubility

Many ionic compounds:

• Dissolve well in water.

Reason:

Water molecules attract and separate the ions.

Examples:

• Sodium chloride
• Potassium nitrate

Many covalent compounds:

• Do not dissolve well in water.

Instead, they often dissolve in organic solvents.

However, there are exceptions.

For example:

Sugar is covalent but dissolves readily in water because its molecules can form hydrogen bonds with water molecules.

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Predicting the Type of Compound

When given an unfamiliar compound:

Step 1

Identify the elements.

Step 2

Ask:

Is it:

Metal + Non-metal?

→ Probably ionic.

Non-metal + Non-metal?

→ Probably covalent.

This simple rule allows chemists to make useful predictions before carrying out experiments.

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Figure 5. The types of elements present provide a useful clue to whether a compound is ionic or covalent.

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Worked Example

Question

Classify each compound as ionic or covalent.

 

Explanation

• Metal + Non-metal → Ionic
• Non-metal + Non-metal → Covalent

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Real-World Connection

Different types of bonding make materials suitable for different purposes. Ionic compounds such as sodium chloride are used in food preservation and as electrolytes in the body because they form ions in solution. Covalent compounds such as plastics, fuels, medicines, and many biological molecules are used because they form stable molecules with a wide variety of useful properties. Engineers and chemists select materials based on how their bonding affects their behaviour.

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Did You Know?

Although both diamond and table salt are hard solids, they are hard for completely different reasons. Diamond is made from a giant covalent network in which every carbon atom is strongly bonded to four others, while table salt is held together by electrostatic attractions between sodium and chloride ions in a giant ionic lattice.

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Key Terms

Anion – A negatively charged ion formed when an atom gains electrons.

Cation – A positively charged ion formed when an atom loses electrons.

Conductivity – The ability of a substance to conduct electricity.

Covalent bond – A chemical bond formed when atoms share pairs of electrons.

Covalent compound – A compound made of atoms joined by covalent bonds.

Electrostatic attraction – The force of attraction between oppositely charged particles.

Ionic bond – The force of attraction between oppositely charged ions.

Ionic compound – A compound made of positive and negative ions held together by ionic bonds.

Ionic lattice – A giant three-dimensional arrangement of alternating positive and negative ions.

Solubility – The ability of a substance to dissolve in a solvent such as water.

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Key Takeaways

• Ionic bonding involves the transfer of electrons, while covalent bonding involves the sharing of electrons.
• Ionic compounds usually form between a metal and a non-metal, while covalent compounds usually form between two non-metals.
• Ionic compounds form giant ionic lattices, whereas covalent compounds usually consist of small molecules.
• Ionic compounds generally have high melting points and conduct electricity when molten or dissolved, while most covalent compounds have lower melting points and do not conduct electricity.
• The physical properties of a substance can be explained by its type of bonding and structure.
• The elements present in a compound often allow you to predict whether it is likely to be ionic or covalent.