What Is a Covalent Bond?

Atoms can join together in several different ways. One important type of chemical bond is a covalent bond.

A covalent bond forms when two atoms share one or more pairs of electrons.

Covalent bonding usually occurs between non-metal atoms. Instead of one atom giving electrons to another, as happens in ionic bonding, the atoms share electrons.

The shared electrons are attracted to the nuclei of both atoms. This attraction holds the atoms together.

Covalent bond = a shared pair of electrons

 

 

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Why Do Atoms Form Covalent Bonds?

Atoms form chemical bonds because bonded arrangements are often more stable than separate atoms.

For many main-group atoms, stability is associated with having a full outer electron shell.

For example:

• hydrogen is stable with 2 electrons in its first shell
• carbon has 4 valence electrons and usually forms 4 covalent bonds
• nitrogen has 5 valence electrons and usually forms 3 covalent bonds
• oxygen has 6 valence electrons and usually forms 2 covalent bonds
• fluorine has 7 valence electrons and usually forms 1 covalent bond

By sharing electrons, each atom can count the shared electrons as part of its outer shell.

This can give both atoms a stable electron arrangement without either atom completely losing or gaining electrons.

Example: Hydrogen

A hydrogen atom has one electron.

Its first electron shell can hold a maximum of two electrons, so an individual hydrogen atom does not have a full shell.

When two hydrogen atoms approach each other, each contributes one electron.

The two electrons are shared:

H:H

The shared pair can also be represented by a line:

H — H

The line represents one covalent bond.

Because the electrons are shared, each hydrogen atom effectively has access to two electrons.

The result is a hydrogen molecule:

H₂

Both hydrogen atoms now have a stable first electron shell.

 

 

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Shared Pairs of Electrons

Each covalent bond contains a pair of shared electrons.

Atoms can share more than one pair of electrons.

Single Bond

One shared pair of electrons forms a single covalent bond.

H — H

Double Bond

Two shared pairs form a double covalent bond.

For example, the two oxygen atoms in an oxygen molecule share two pairs of electrons:

O = O

Triple Bond

Three shared pairs form a triple covalent bond.

The two nitrogen atoms in a nitrogen molecule share three pairs:

N ≡ N

Therefore:

• 1 shared pair → single bond
• 2 shared pairs → double bond
• 3 shared pairs → triple bond

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Which Atoms Form Covalent Bonds?

Covalent bonds form mainly between non-metal atoms.

Common non-metals that form covalent bonds include:

• hydrogen (H)
• carbon (C)
• nitrogen (N)
• oxygen (O)
• fluorine (F)
• phosphorus (P)
• sulfur (S)
• chlorine (Cl)

These atoms can form bonds with atoms of the same element or with different non-metals.

For example:

Many covalently bonded substances exist as separate groups of atoms called molecules.

Example: Water

Water provides a good example of how atoms use covalent bonding to achieve stable electron arrangements.

An oxygen atom has 6 valence electrons.

It needs two additional electrons to complete its outer shell.

Each hydrogen atom has 1 electron and needs one additional electron to fill its first shell.

Oxygen therefore shares electrons with two hydrogen atoms.

This produces:

H — O — H

The molecule has two O – H covalent bonds.

Through sharing:

• oxygen achieves a stable outer arrangement
• each hydrogen achieves a stable first shell

The result is a water molecule, H₂O.

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Example: Methane

Carbon provides another important example.

A carbon atom has 4 valence electrons. It can form four covalent bonds by sharing electrons with other atoms.

In methane, carbon shares electrons with four hydrogen atoms.

The formula is:

A simple structural representation is:

       H
       |
   H – C – H
       |
       H

Each line represents one shared pair of electrons.

Carbon forms four covalent bonds, giving it a stable outer electron arrangement.

Each hydrogen forms one covalent bond, giving it a stable first shell.

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Covalent Bonding vs Ionic Bonding

Covalent and ionic bonding both allow atoms to form more stable electron arrangements, but they do this in different ways.

The most important difference is what happens to the valence electrons.

Covalent bonding

SHARING electrons

Ionic bonding

TRANSFERRING electrons

This distinction is extremely important.

In a covalent bond, electrons are not completely transferred from one atom to another. They are shared between the bonded atoms.

Comparing Water and Sodium Chloride

Consider two familiar substances.

Water – H₂O

Hydrogen and oxygen are both non-metals.

They share electrons.

Therefore, the bonding is covalent.

Sodium Chloride – NaCl

Sodium is a metal and chlorine is a non-metal.

Sodium transfers an electron to chlorine, producing Na⁺ and Cl⁻ ions.

Therefore, the bonding is ionic.

A useful starting rule is:

non-metal + non-metal → usually covalent

metal + non-metal → usually ionic

Covalent Bonds and Stable Electron Arrangements

The purpose of electron sharing becomes clearer when we look at the outer electron shells.

Consider oxygen.

An oxygen atom has the electron arrangement:

2, 6

Its outer shell contains 6 electrons.

When oxygen forms two covalent bonds, it shares two additional electrons with other atoms. This allows oxygen to achieve an outer-shell arrangement associated with 8 electrons.

This is often described using the octet rule.

The Octet Rule

Many main-group atoms tend to form bonds that give them access to eight electrons in their outer shell.

However, hydrogen is an important exception. Its first shell is full with only two electrons.

The octet rule is a useful introductory model, but it is not a rule that every atom and compound follows.

Real-World Connection

Covalent bonds are found in many substances that are essential to life.

For example:

• water (H₂O) contains covalent bonds
• oxygen (O₂) contains covalent bonds
• carbon dioxide (CO₂) contains covalent bonds
• glucose (C₆H₁₂O₆) contains many covalent bonds
• DNA contains many covalently bonded atoms
• proteins contain networks of covalent bonds

Carbon is particularly important because each carbon atom can form four covalent bonds. This allows carbon atoms to form chains, rings, and complex structures.

This ability is one reason carbon is the foundation of the enormous variety of molecules found in living organisms.

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Did You Know?

The electrons in a covalent bond do not necessarily have to be shared equally.

Some atoms attract shared electrons more strongly than others. This property is called electronegativity.

For example, oxygen attracts shared electrons more strongly than hydrogen does. As a result, the electrons in the O–H bonds of water spend more time closer to the oxygen atom.

This unequal sharing gives water many of its unusual and important properties.

You will encounter this idea again when studying polar covalent bonds.

Key Vocabulary

Covalent bond – a chemical bond formed when atoms share a pair of electrons.

Shared pair – two electrons shared between bonded atoms.

Valence electron – an electron in the outermost occupied shell of an atom.

Molecule – a discrete group of atoms held together by covalent bonds.

Single bond – a covalent bond containing one shared pair of electrons.

Double bond – a covalent bond containing two shared pairs of electrons.

Triple bond – a covalent bond containing three shared pairs of electrons.

Stable electron arrangement – a lower-energy electron arrangement often associated, in simple main-group models, with a full outer shell.

Octet rule – the useful introductory idea that many main-group atoms form bonds that give them access to eight outer-shell electrons.

Key Takeaways

• Covalent bonds form when atoms share electrons.
• Covalent bonding usually occurs between non-metal atoms.
• A covalent bond contains a shared pair of electrons.
• Atoms form covalent bonds because bonding can produce a more stable, lower-energy electron arrangement.
• Sharing electrons can allow atoms to achieve a full outer electron shell in simple bonding models.
• One shared pair produces a single bond.
• Two shared pairs produce a double bond.
• Three shared pairs produce a triple bond.
• Covalent bonding involves sharing electrons, while ionic bonding involves electron transfer and attraction between ions.
• Common covalent substances include H₂, O₂, H₂O, CO₂, CH₄, and NH₃.