Introduction

Carbon is the foundation of organic chemistry because it can form an extraordinary variety of stable compounds. It appears in simple substances such as methane, in biological molecules such as glucose and DNA, and in synthetic materials such as plastics and medicines.

Carbon’s versatility comes from several linked properties: it has four valence electrons, it forms strong covalent bonds, it can bond to itself repeatedly, and it can make single, double, or triple bonds. Together, these properties allow carbon atoms to build chains, branches, rings, and complex three-dimensional structures.

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Carbon’s Electron Structure

Carbon has an atomic number of 6, which means a neutral carbon atom has six electrons.

Its electron arrangement is:

$2,4$2,4

This means carbon has:

• Two electrons in its first shell.
• Four electrons in its outer shell.

The four outer electrons are called valence electrons because they take part in chemical bonding.

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Why Carbon Forms Four Bonds

Atoms are generally more stable when their outer electron shells are filled. Carbon would need to gain or lose four electrons to form a full outer shell, but doing either would require a large amount of energy.

Instead, carbon usually shares electrons with other atoms.

By sharing four pairs of electrons, carbon can form up to four covalent bonds.

For example, in methane:

$\mathrm{CH_4}$CH4​

one carbon atom forms four single covalent bonds with four hydrogen atoms.

This bonding capacity is called tetravalency.

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Tetravalency

Tetravalency means that carbon can form four covalent bonds.

These bonds may be formed with:

• Other carbon atoms.
• Hydrogen.
• Oxygen.
• Nitrogen.
• Sulfur.
• Halogens such as chlorine.

Because carbon can bond with many elements in different arrangements, it can form an enormous variety of compounds.

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Strong Covalent Bonds

Carbon forms strong covalent bonds because carbon atoms are small.

The small size of the atom allows the shared electrons to remain strongly attracted to both nuclei.

Carbon can form strong bonds such as:

• Carbon–carbon bonds.
• Carbon–hydrogen bonds.
• Carbon–oxygen bonds.
• Carbon–nitrogen bonds.

These bonds are stable enough to form large molecules but can still participate in chemical reactions.

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Single Carbon–Carbon Bonds

A single bond forms when two carbon atoms share one pair of electrons.

It is represented by one line:

$\mathrm{C-C}$C−C

An example is ethane:

$\mathrm{C_2H_6}$C2​H6​

Single carbon–carbon bonds allow atoms to rotate relative to one another, giving molecules flexibility.

Compounds containing only carbon–carbon single bonds are described as saturated.

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Double Carbon–Carbon Bonds

A double bond forms when two carbon atoms share two pairs of electrons.

It is represented by two lines:

$\mathrm{C=C}$C=C

An example is ethene:

$\mathrm{C_2H_4}$C2​H4​

Double bonds are:

• Shorter than single bonds.
• Stronger than single bonds overall.
• Less able to rotate freely.
• More chemically reactive than single carbon–carbon bonds.

Compounds containing carbon–carbon double bonds are unsaturated.

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Triple Carbon–Carbon Bonds

A triple bond forms when two carbon atoms share three pairs of electrons.

It is represented by three lines:

$\mathrm{C \equiv C}$C≡C

An example is ethyne:

$\mathrm{C_2H_2}$C2​H2​

Triple bonds are:

• Shorter than single and double bonds.
• Very strong.
• Rigid.
• Chemically reactive.

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Comparing Carbon–Carbon Bonds

As bond order increases:

• Bond length decreases.
• Bond strength generally increases.
• Molecular rotation becomes more restricted.

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What Is Catenation?

Catenation is the ability of an element to bond repeatedly with atoms of the same element.

Carbon shows an exceptional ability to bond with other carbon atoms.

It can form:

• Straight chains.
• Branched chains.
• Rings.
• Long polymers.
• Complex three-dimensional structures.

For example:

Straight chain:

$\mathrm{C-C-C-C}$C−C−C−C

Branched structure:

$\begin{array}{c} \mathrm{\ \ C}\\ \mathrm{\ \ |}\\ \mathrm{C-C-C} \end{array}$  C  ∣C−C−C​

Ring structure:

A closed loop of carbon atoms.

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Why Carbon Shows Strong Catenation

Carbon is especially good at catenation because:

• Carbon–carbon bonds are strong.
• Carbon atoms are small.
• Carbon can form single, double, and triple bonds.
• Carbon can form stable chains of many different lengths.

Other elements, such as silicon, can also bond to themselves, but their chains are generally less stable and less varied than carbon chains.

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Chains, Branches, and Rings

Carbon skeletons form the backbone of organic molecules.

Straight Chains

Carbon atoms join in a continuous line.

Examples include many alkanes.

Branched Chains

One or more carbon groups extend from the main chain.

Branching changes properties such as boiling point and shape.

Rings

Carbon atoms join to form closed structures.

Examples include:

• Cyclohexane.
• Benzene.
• Many biological molecules.

Different carbon skeletons can produce compounds with very different properties.

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Carbon Bonds with Other Elements

Carbon does not only bond with itself.

It commonly bonds with:

• Hydrogen in hydrocarbons.
• Oxygen in alcohols and carboxylic acids.
• Nitrogen in amino acids and proteins.
• Halogens in many industrial compounds.
• Sulfur in some biological molecules.

Adding different atoms or groups of atoms creates functional groups, which give organic compounds their characteristic reactions and properties.

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The Diversity of Organic Molecules

Carbon’s tetravalency, strong bonding, multiple bond types, and catenation produce a huge range of possible molecular structures.

Organic molecules may differ in:

• Chain length.
• Branching.
• Ring formation.
• Bond type.
• Functional groups.
• Three-dimensional arrangement.

Even compounds with the same molecular formula can sometimes have different structures. These compounds are called isomers.

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Carbon in Biological Molecules

Carbon’s versatility makes complex life possible.

It forms the skeletons of:

• Carbohydrates.
• Lipids.
• Proteins.
• DNA and RNA.

These molecules can store energy, build structures, carry information, and control chemical reactions in living organisms.

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Carbon in Industry

Carbon compounds are also central to modern industry.

They are used to produce:

• Fuels.
• Plastics.
• Pharmaceuticals.
• Dyes.
• Detergents.
• Synthetic fibres.
• Solvents.

By changing the arrangement of carbon atoms and functional groups, chemists can design compounds with specific properties.

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Worked Example

Consider the three molecules:

$\mathrm{C_2H_6,\ C_2H_4,\ C_2H_2}$C2​H6​, C2​H4​, C2​H2​

Question

Identify the type of carbon–carbon bond in each molecule.

Solution

Ethane,

$\mathrm{C_2H_6}$

C2​H6​:

The carbon atoms share one pair of electrons.

$\mathrm{C-C}$C−C

It contains a single bond.

Ethene,

$\mathrm{C_2H_4}$

C2​H4​:

The carbon atoms share two pairs of electrons.

$\mathrm{C=C}$C=C

It contains a double bond.

Ethyne,

$\mathrm{C_2H_2}$

C2​H2​:

The carbon atoms share three pairs of electrons.

$\mathrm{C \equiv C}$C≡C

It contains a triple bond.

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Real-World Connections

Carbon’s bonding versatility explains why very different materials can all be carbon-based.

For example:

• Diamond consists of carbon atoms bonded in a rigid three-dimensional network.
• Graphite consists of carbon atoms arranged in layers.
• Plastics contain long carbon-based polymer chains.
• Medicines contain carefully designed carbon skeletons and functional groups.
• DNA contains a carbon-based backbone supporting genetic information.

The arrangement of atoms is just as important as the elements present.

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Did You Know?

• Diamond and graphite are both made entirely of carbon, but their different bonding arrangements give them completely different properties.
• Some synthetic polymers contain chains made of tens of thousands of carbon atoms.
• Carbon can form more known compounds than all other elements combined.

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Key Terms

Valence Electron — An electron in the outer shell of an atom that participates in bonding.

Tetravalency — The ability of carbon to form four covalent bonds.

Catenation — The ability of an element to form bonds with other atoms of the same element.

Single Bond — A covalent bond involving one shared pair of electrons.

Double Bond — A covalent bond involving two shared pairs of electrons.

Triple Bond — A covalent bond involving three shared pairs of electrons.

Carbon Skeleton — The chain, branch, or ring of carbon atoms forming the framework of an organic molecule.

Isomer — A compound with the same molecular formula as another compound but a different arrangement of atoms.

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Key Takeaways

• Carbon has four valence electrons and normally forms four covalent bonds.
• Carbon’s four-bond capacity is known as tetravalency.
• Carbon can form single, double, and triple bonds with other carbon atoms.
• Strong carbon–carbon bonds allow carbon to undergo extensive catenation.
• Carbon atoms can form straight chains, branched chains, rings, and large three-dimensional structures.
• Carbon also bonds readily with hydrogen, oxygen, nitrogen, sulfur, and halogens.
• These properties explain the enormous diversity of organic compounds found in living organisms and modern industry.

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Suggested Images

Suggested placement:

1. After “Why Carbon Forms Four Bonds” – A shell or Lewis diagram showing carbon’s four valence electrons and four covalent bonds.
2. After “Comparing Carbon–Carbon Bonds” – Side-by-side structural models of ethane, ethene, and ethyne, highlighting single, double, and triple bonds.
3. After “What Is Catenation?” – Diagram showing straight, branched, and ring-shaped carbon skeletons.
4. After “The Diversity of Organic Molecules” – Collage or molecular diagram showing hydrocarbons, proteins, carbohydrates, DNA, and polymers.
5. After “Real-World Connections” – Comparison of diamond and graphite structures, demonstrating how different carbon arrangements produce different properties.