Introduction

Not all atoms form compounds by transferring electrons. Many substances, including water, oxygen, carbon dioxide, methane, and the molecules found in living organisms, are formed when atoms share electrons. This type of bonding is called covalent bonding.

Covalent bonding occurs mainly between non-metal atoms. By sharing electrons, each atom can achieve a full outer electron shell, becoming more stable. Covalent bonds are responsible for forming millions of different molecules, from the oxygen we breathe to the DNA inside our cells.

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What Is Covalent Bonding?

A covalent bond is a shared pair of electrons between two atoms.

Instead of transferring electrons, the atoms share them.

Covalent bonding usually occurs between:

• Non-metal and non-metal atoms.

Examples include:

• Hydrogen (H₂)
• Oxygen (O₂)
• Water (H₂O)
• Carbon dioxide (CO₂)
• Methane (CH₄)

By sharing electrons, both atoms achieve more stable outer electron shells.

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Figure 1. Covalent bonds form when non-metal atoms share pairs of electrons.

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Why Do Atoms Share Electrons?

Most atoms become more stable when their outer electron shell is full.

Rather than gaining or losing electrons, non-metals often share electrons.

For example:

Hydrogen has:

• 1 electron.

By sharing one electron with another hydrogen atom, both atoms achieve a full first shell containing 2 electrons.

Oxygen shares electrons so that each oxygen atom effectively has 8 electrons in its outer shell.

This stable arrangement is often called the octet rule (or the duet rule for hydrogen).

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Lewis Structures

A Lewis structure shows:

• The chemical symbols.
• The shared pairs of electrons (bonds).
• The unshared (lone) pairs of electrons.

Each shared pair represents one covalent bond.

Examples:

Hydrogen (H₂)

H—H

One shared pair of electrons.

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Chlorine (Cl₂)

Cl—Cl

Each chlorine atom shares one electron.

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Water (H₂O)

Oxygen forms two covalent bonds.

H—O—H

Oxygen also has two lone pairs of electrons.

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Figure 2. Lewis structures show shared electron pairs (bonds) and lone pairs around atoms.

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Dot-and-Cross Diagrams

A dot-and-cross diagram shows:

• Which electrons come from each atom.
• Which electrons are shared.

Dots (•) represent electrons from one atom.

Crosses (×) represent electrons from the other atom.

The shared pair contains:

• One dot.
• One cross.

Dot-and-cross diagrams help illustrate exactly how covalent bonds form.

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Figure 3. Dot-and-cross diagrams show how atoms share electrons to form covalent bonds.

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How Many Bonds Do Common Atoms Form?

Different non-metals usually form a predictable number of covalent bonds.

For example:

Carbon has four valence electrons.

It usually shares four pairs of electrons, allowing it to form four covalent bonds.

This ability allows carbon to form millions of different compounds.

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Single, Double, and Triple Bonds

Atoms may share:

• One pair of electrons.
• Two pairs of electrons.
• Three pairs of electrons.

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Single Bond

One shared pair.

Example:

H—H

or

Cl—Cl

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Double Bond

Two shared pairs.

Example:

O=O

Each oxygen atom shares two pairs of electrons.

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Triple Bond

Three shared pairs.

Example:

N≡N

Each nitrogen atom shares three pairs of electrons.

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Figure 4. Covalent bonds may involve one, two, or three shared pairs of electrons.

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Properties of Covalent Substances

The type of bonding helps explain the properties of covalent substances.

Low Melting and Boiling Points

Many covalent substances consist of small molecules.

The forces between these molecules are relatively weak.

As a result, many have low melting and boiling points.

Examples:

• Oxygen
• Carbon dioxide
• Water (compared with ionic compounds)

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Poor Electrical Conductors

Most covalent substances do not conduct electricity because they contain:

• No free ions.
• No free-moving electrons.

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Can Exist as Solids, Liquids, or Gases

Examples:

Solid:

• Iodine

Liquid:

• Water

Gas:

• Oxygen
• Carbon dioxide

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Some Exceptions

Some giant covalent structures behave differently.

Examples:

• Diamond
• Graphite
• Silicon dioxide

These have:

• Very high melting points.

Graphite also conducts electricity because it contains mobile electrons.

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Figure 5. Most covalent substances are made of small molecules, but giant covalent structures such as diamond and graphite have very different properties.

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Ionic and Covalent Bonding Compared

Both types of bonding help atoms achieve stable electron arrangements, but they do so in different ways.

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Why Covalent Bonding Is Important

Covalent compounds are found everywhere.

Examples include:

• Water
• Oxygen
• Carbon dioxide
• Sugars
• Proteins
• DNA
• Plastics
• Medicines

Almost every molecule found in living organisms is held together by covalent bonds.

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Worked Example

Question

Explain how a water molecule (H₂O) forms.

Solution

1. Oxygen has 6 valence electrons and needs 2 more to complete its outer shell.
2. Each hydrogen atom has 1 electron and needs 1 more to fill its first shell.
3. Oxygen shares one pair of electrons with each hydrogen atom.
4. Two single covalent bonds are formed.
5. Oxygen now has 8 electrons in its outer shell, and each hydrogen has 2 electrons in its first shell.

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Real-World Connection

The molecules that make up living organisms are held together by covalent bonds. Water, DNA, proteins, carbohydrates, and lipids all depend on covalent bonding for their structure and function. Chemists also design new covalent compounds to produce medicines, plastics, synthetic fibres, fuels, and advanced materials used in electronics and aerospace engineering.

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Did You Know?

The human body contains trillions of water molecules, and each water molecule is held together by two covalent bonds between oxygen and hydrogen atoms. Without these covalent bonds, water—the substance essential for all known life—could not exist.

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Key Terms

Covalent bond – A chemical bond formed when two atoms share one or more pairs of electrons.

Double bond – A covalent bond in which two pairs of electrons are shared.

Dot-and-cross diagram – A diagram showing which electrons each atom contributes to a covalent bond.

Lewis structure – A diagram showing atoms, shared electron pairs (bonds), and lone pairs of electrons.

Lone pair – A pair of valence electrons that is not involved in bonding.

Molecule – A group of atoms held together by covalent bonds.

Octet rule – The tendency of atoms to achieve eight electrons in their outer shell through bonding.

Single bond – A covalent bond involving one shared pair of electrons.

Triple bond – A covalent bond involving three shared pairs of electrons.

Valence electron – An electron in the outermost shell of an atom that participates in chemical bonding.

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Key Takeaways

• Covalent bonding occurs when non-metal atoms share electrons to achieve stable outer electron shells.
• Lewis structures and dot-and-cross diagrams show how electrons are shared in molecules.
• Different atoms typically form a predictable number of covalent bonds based on their valence electrons.
• Covalent bonds can be single, double, or triple, depending on the number of shared electron pairs.
• Most covalent substances have low melting and boiling points and do not conduct electricity, although giant covalent structures such as diamond and graphite are important exceptions.
• Covalent bonding is responsible for forming the molecules that make up living organisms and countless everyday materials.