Covalent Bonding
This lesson covers covalent bonding — the sharing of electron pairs between non-metal atoms — as required by the Edexcel GCSE Chemistry specification (1CH0). You need to understand how covalent bonds form, draw dot-and-cross diagrams for a range of molecules, and recognise single, double and triple bonds.
What Is Covalent Bonding?
A covalent bond is a shared pair of electrons between two non-metal atoms. Covalent bonding occurs between non-metals only.
Each atom in the bond contributes one electron to the shared pair (in a single bond). The shared pair of electrons is attracted to the nuclei of both atoms, holding the atoms together.
Why do atoms form covalent bonds?
- Non-metal atoms need to gain electrons to achieve a full outer shell.
- When two non-metal atoms are close together, instead of one atom taking electrons from the other, they share electrons.
- By sharing, both atoms can achieve the electronic structure of a noble gas (full outer shell).
Exam Tip: A covalent bond is a "shared pair of electrons." This is the definition the examiner expects. Don't just say "sharing electrons" — specify that it's a pair.
Single, Double and Triple Bonds
Atoms can share more than one pair of electrons:
| Bond Type | Electron Pairs Shared | Representation | Example |
|---|
| Single bond | 1 pair (2 electrons) | — | H–H, H–Cl |
| Double bond | 2 pairs (4 electrons) | = | O=O, C=O |
| Triple bond | 3 pairs (6 electrons) | ≡ | N≡N |
The more electron pairs that are shared, the stronger and shorter the bond.
graph LR
A["Single Bond<br/>1 shared pair<br/>Longest & weakest"] --> B["Double Bond<br/>2 shared pairs<br/>Shorter & stronger"]
B --> C["Triple Bond<br/>3 shared pairs<br/>Shortest & strongest"]
Dot-and-Cross Diagrams for Covalent Molecules
Dot-and-cross diagrams show the outer shell electrons of each atom. Electrons from one atom are shown as dots (•) and electrons from the other atom as crosses (×). The shared pair sits in the overlap between the two atoms.
Hydrogen (H₂) — Single Bond
- Each hydrogen atom has 1 electron in its outer shell.
- Each hydrogen needs 1 more electron to fill its outer shell (like helium, which has 2).
- They share 1 pair of electrons.
- Diagram: each H has 1 dot/cross in the overlap. Each H atom now has 2 electrons in its outer shell.
- Displayed formula: H–H
Hydrogen Chloride (HCl) — Single Bond
- Hydrogen has 1 electron in its outer shell.
- Chlorine has 7 electrons in its outer shell.
- They share 1 pair of electrons.
- Diagram: the shared pair (1 dot from H, 1 cross from Cl) sits between H and Cl. Chlorine also has 3 lone pairs (non-bonding pairs) of electrons.
- Displayed formula: H–Cl
Water (H₂O) — Two Single Bonds
- Oxygen has 6 electrons in its outer shell — needs 2 more.
- Each hydrogen has 1 electron — each needs 1 more.
- Oxygen shares 1 pair with each of two hydrogen atoms.
- Diagram: oxygen has 2 bonding pairs (one with each H) and 2 lone pairs.
- Displayed formula: H–O–H
Ammonia (NH₃) — Three Single Bonds
- Nitrogen has 5 electrons in its outer shell — needs 3 more.
- Each hydrogen has 1 electron — each needs 1 more.
- Nitrogen shares 1 pair with each of three hydrogen atoms.
- Diagram: nitrogen has 3 bonding pairs and 1 lone pair.
- Displayed formula: H–N with two more H atoms bonded (trigonal pyramidal shape).
Methane (CH₄) — Four Single Bonds
- Carbon has 4 electrons in its outer shell — needs 4 more.
- Each hydrogen has 1 electron — each needs 1 more.
- Carbon shares 1 pair with each of four hydrogen atoms.
- Diagram: carbon has 4 bonding pairs and no lone pairs.
- Displayed formula: H bonded to C from four directions.
Oxygen (O₂) — Double Bond
- Each oxygen has 6 electrons in its outer shell — each needs 2 more.
- The two oxygen atoms share 2 pairs of electrons (a double bond).
- Diagram: each oxygen has 2 bonding pairs in the overlap and 2 lone pairs.
- Displayed formula: O=O
Carbon Dioxide (CO₂) — Two Double Bonds
- Carbon has 4 electrons in its outer shell — needs 4 more.
- Each oxygen has 6 electrons — each needs 2 more.
- Carbon shares 2 pairs with each of two oxygen atoms.
- Diagram: carbon has 4 bonding pairs (2 with each O), no lone pairs. Each oxygen has 2 bonding pairs and 2 lone pairs.
- Displayed formula: O=C=O
Nitrogen (N₂) — Triple Bond
- Each nitrogen has 5 electrons in its outer shell — each needs 3 more.
- The two nitrogen atoms share 3 pairs of electrons (a triple bond).
- Diagram: each nitrogen has 3 bonding pairs in the overlap and 1 lone pair.
- Displayed formula: N≡N
Exam Tip: In your dot-and-cross diagram, you must show all outer shell electrons, not just the bonding pairs. Don't forget the lone pairs! The examiner will check for these.
Summary of Molecules
| Molecule | Formula | Bonds | Bonding Pairs | Lone Pairs (on central atom) |
|---|
| Hydrogen | H₂ | 1 single | 1 | 0 |
| Hydrogen chloride | HCl | 1 single | 1 | 3 (on Cl) |
| Water | H₂O | 2 single | 2 | 2 (on O) |
| Ammonia | NH₃ | 3 single | 3 | 1 (on N) |
| Methane | CH₄ | 4 single | 4 | 0 |
| Oxygen | O₂ | 1 double | 2 | 2 (on each O) |
| Carbon dioxide | CO₂ | 2 double | 4 | 2 (on each O) |
| Nitrogen | N₂ | 1 triple | 3 | 1 (on each N) |
Bond Length and Bond Energy
- Bond length is the distance between the nuclei of two bonded atoms.
- Bond energy is the energy required to break one mole of a particular bond in a gaseous molecule.
| Bond Type | Relative Bond Length | Relative Bond Energy |
|---|
| Single (e.g. C–C) | Longest | Lowest |
| Double (e.g. C=C) | Shorter | Higher |
| Triple (e.g. C≡C) | Shortest | Highest |
As the number of shared electron pairs increases:
- The atoms are pulled closer together (shorter bond length).
- More energy is needed to break the bond (higher bond energy).
Key Points
- A covalent bond is a shared pair of electrons between two non-metal atoms.
- Single bonds share 1 pair, double bonds share 2 pairs, triple bonds share 3 pairs.
- Dot-and-cross diagrams must show all outer shell electrons including lone pairs.
- You must be able to draw diagrams for: H₂, HCl, H₂O, NH₃, CH₄, O₂, CO₂, N₂.
- More shared pairs = shorter bond length and higher bond energy.
- Covalent bonding occurs between non-metals only.
Exam Tip: Learn the dot-and-cross diagrams for all eight molecules listed above. These come up very frequently in exam questions.
Practice Questions
- What is meant by the term "covalent bond"?
- Draw a dot-and-cross diagram for a molecule of water (H₂O).
- Draw a dot-and-cross diagram for a molecule of carbon dioxide (CO₂).
- Explain why a nitrogen molecule (N₂) has a triple bond.
- State whether a double bond is stronger or weaker than a single bond, and explain why.
Worked Example: Drawing Dot-and-Cross Diagrams from Scratch