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Covalent bonding and electron diagrams

Count shared and lone electron pairs, draw dot-and-cross diagrams, and deduce single, double and triple covalent bonds.

  • SEC G3 Pure Chemistry 2027

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Covalent bonding questions become easier when you picture two atoms sharing electrons. Start with the syllabus idea—a shared pair of electrons—then use the attraction between that pair and both nuclei when a question asks you to explain the bond.

Understand covalent bonding

A covalent bond is formed when two atoms share a pair of electrons. The shared pair is attracted to both positively charged nuclei, holding the atoms together.

It usually occurs between non-metal atoms.

Key ideas

  • Covalent bonding involves electron sharing (not transfer).
  • A single bond = 1 shared pair of electrons.
  • A double bond = 2 shared pairs of electrons.
  • A triple bond = 3 shared pairs of electrons.
  • Lone pairs belong to one atom; shared pairs are counted in the outer shells of both bonded atoms.
  • Dots and crosses track where electrons came from. All electrons are the same kind of particle.

Count shared and lone pairs

Count pairs without losing electrons
  • A shared pair contains two electrons and is attracted to both nuclei.
  • Count a shared pair for both bonded atoms, but count its electrons only once in the molecule’s total.
  • Show every lone pair as well as every shared pair.

Build a shared-electron model

In the molecules shown here, sharing gives hydrogen access to two outer electrons and carbon, nitrogen and oxygen access to eight. This is a useful electron-counting model, rather than a literal picture of electron paths. The shared electrons are attracted to both nuclei.

Shared pairs and lone pairs in hydrogen, oxygen, nitrogen, water, methane and carbon dioxideHydrogen has one shared pair. Oxygen has two shared pairs and two lone pairs on each oxygen. Nitrogen has three shared pairs and one lone pair on each nitrogen. Water has two O–H shared pairs and two lone pairs on oxygen. Methane has four C–H shared pairs and no lone pairs on carbon. Carbon dioxide has two shared pairs in each C–O bond and two lone pairs on each oxygen. Dots and crosses distinguish electron origins, not kinds of electron.H₂: one single bondHH•×2 valence electrons in total• left atom · × right atomO₂: one double bondOO•×•×••••××××12 valence electrons in total• left atom · × right atomN₂: one triple bondNN•×•×•×••××10 valence electrons in total• left atom · × right atomH₂O: two single bondsOHHו•×••••8 valence electrons in total• oxygen · × hydrogenCH₄: four single bondsCHHHH•×•×•×•×8 valence electrons in total• carbon · × hydrogenCO₂: two double bondsCOO•×•×•×•×××××××××16 valence electrons in total• carbon · × oxygen
Dot-and-cross diagrams: a shared pair lies in both atoms’ outer shells, while a lone pair belongs to one atom. In these examples each bond pair has one electron from each bonded atom. All valence electrons are shown once. The circles and positions are an electron-counting model, not electron paths, atom sizes or three-dimensional molecular shapes.

Count single, double and triple bonds

Bond typeShared pairsExample
Single1H₂, HCl, CH₄
Double2O₂, CO₂ (two C = O double bonds)
Triple3N₂

A bond type does not determine the whole structure

Covalent bonds can hold together a small molecule, such as CH₄, or extend through a giant network, such as diamond. A covalent bond names the electron-sharing interaction; simple molecular and giant covalent describe how far the structure extends.

Use Molecular structures and properties to explain physical changes and conductivity, then Giant covalent structures to compare diamond, graphite and silicon dioxide.

Misconceptions to check

  • Writing “covalent bonding = transfer of electrons” (wrong). Covalent = sharing.
  • Using “intermolecular forces” when you mean “covalent bonds” (or vice versa).
  • Not stating the number of shared pairs for double/triple bonds.

Check your electron diagram

Match the wording to the question

For formation, state that two atoms share a pair of electrons. If asked what holds the atoms together, add that the shared pair is attracted to both nuclei.

Check the electron total

Add the valence electrons supplied by all the atoms. Your diagram must show the same total, including the electrons in lone pairs. For water: 6 + 1 + 1 = 8 electrons, arranged as two shared pairs and two lone pairs on oxygen.

Worked examples

Modelled example 1

Describe Covalent Bonding (Methane)

Core

Problem

Describe the bonding in a methane molecule, CH₄.

Study the worked solution

  1. Count valence electrons

    Method

    Identify the electrons available for bonding.

    Reason

    Carbon has four valence electrons; each hydrogen has one.

    Working

    C: 4 valence electrons; 4 ×H: one each.

  2. Form shared pairs

    Method

    Share one pair between carbon and each of four hydrogen atoms.

    Reason

    Each shared pair is attracted to both nuclei and forms one single covalent bond.

    Working

    Four C-H shared pairs → four single covalent bonds.

  3. Check outer shells

    Method

    Count each shared pair for both bonded atoms.

    Reason

    Carbon then has an octet and each hydrogen has a duplet.

    Working

    CH₄ is a simple molecule with four C-H covalent bonds.

Guided practice 2

Identify Bond Type From the Elements

About 5 min

Problem

Predict the bonding type in (a) MgO and (b) CO₂. Give a particle-level reason for each.

Classify each element before choosing the bond

MgO bonding
CO2 bonding

Hints

Hint 1: classify the elements

Magnesium is a metal; carbon and oxygen are non-metals.

Hint 2: connect element types to electrons

Metal plus non-metal usually transfers electrons; non-metal plus non-metal shares them.

View solution step by step

  1. Analyse magnesium oxide

    Method

    Apply the metal–non-metal pattern.

    Reason

    Electrons transfer from magnesium to oxygen, forming oppositely charged ions.

    Working

    MgO has ionic bonding.

  2. Analyse carbon dioxide

    Method

    Apply the non-metal–non-metal pattern.

    Reason

    Carbon and oxygen atoms share electron pairs rather than forming a metal cation.

    Working

    CO₂ has covalent bonding.

Continue this structure–property explanation in Molecular structures and properties.

Continue this structure–property explanation in Molecular structures and properties.

Challenge 3

Double vs Single Bonds (CO2)

Minimal support

Electron-count transfer

Carbon has four valence electrons and each oxygen has six. Deduce why the structure of carbon dioxide is (O = C = O) rather than using two single C-O bonds.

Count shared pairs needed for each octet

Pairs shared with each oxygen
Each C-O bond

Hints

Hint 1: start from oxygen

Each oxygen begins with six valence electrons and needs access to two more for an octet.

Hint 2: translate pairs into bond type

One shared pair is a single bond; two shared pairs between the same atoms form a double bond.

View solution step by step

  1. Determine each oxygen's need

    Method

    Compare six valence electrons with an octet.

    Reason

    Each oxygen needs access to two additional electrons.

    Working

    8-6 = 2 electrons needed by each O atom.

  2. Determine carbon's need

    Method

    Distribute carbon’s four bonding electrons across two oxygen atoms.

    Reason

    Sharing two pairs with each oxygen gives all three atoms an octet.

    Working

    2 shared pairs per C-O connection × 2 oxygen atoms.

  3. State the bond multiplicity

    Working

    Two shared pairs form each C=O double bond, giving (O = C = O).

Try these independently

Try this property comparison in Molecular structures and properties.

Try this property comparison in Molecular structures and properties.

Mind stretcher 1: Deduce the electron pairs in hydrogen sulfideExtension

Question: Sulfur has six valence electrons and hydrogen has one. In H₂S, each hydrogen reaches a full first shell and sulfur reaches an octet. Deduce the shared pairs and lone pairs, and check the total electrons.

Show answer

There are two S–H single bonds, each containing one electron from sulfur and one from hydrogen. Sulfur also has two lone pairs. Each H has access to two electrons; S has access to eight. The molecule contains 6 + 2(1) = 8 valence electrons: four in shared pairs and four in lone pairs.

Practise and check

Topic check

See what you know across this topic, then go back to anything you got wrong.

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Syllabus and review details

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