Build covalent and coordinate bonds
Explain shared-pair covalent bonding, coordinate donation, named molecules and expanded octets for Period 3 compounds.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the supplied Pauling values when given, show particle and bond reasoning, and distinguish intramolecular bonds from intermolecular forces. Written practice is self-checked
Before you begin
- Read element symbols and simple molecular formulae; use the Periodic Table.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is shared in a covalent bond?
What you will learn
- Define covalent bonding as attraction involving a shared pair
- Describe covalent bonds in syllabus molecules
- Explain coordinate bonding and Period 3 expanded octets
A covalent bond is attraction to a shared pair
Covalent bonding is the electrostatic attraction between the nuclei of two atoms and a shared pair of electrons. Single, double and triple bonds contain one, two and three shared pairs respectively.
Use this model for H₂, O₂, N₂, Cl₂, HCl, CO₂, NH₃, CH₄, C₂H₆ and C₂H₄. Lewis structures count valence electrons, show shared pairs between bonded atoms and display remaining lone pairs where relevant.
A coordinate bond starts with a donated lone pair
In coordinate (dative covalent) bonding, both electrons in the shared pair come from one atom. Ammonia can donate its nitrogen lone pair to H⁺ to form NH₄⁺. Once formed, that bond is a shared pair; the arrow records how it formed, not a permanently different bond type.
Coordinate bonding also occurs in Al₂Cl₆. Period 3 atoms can be represented with an expanded octet in SO₂, PCl₅ and SF₆ where the syllabus names these examples. Follow the stated electron-counting convention and do not force every atom to obey a four-pair octet in these compounds.
Pause and explain
Where do both electrons in a coordinate bond come from initially?
Worked example
Explain how NH₃ and HCl gases can form NH₄⁺ and identify the coordinate bond.
Show the worked solution
- Nitrogen in NH₃ has a lone pair.
- Hydrogen chloride can provide H⁺.
- Nitrogen donates its lone pair to H⁺, forming a shared pair and the ammonium ion.
Answer NH₃ + H⁺ → NH₄⁺. One N–H shared pair forms when nitrogen donates both electrons; it is a coordinate bond on formation.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the bonds in CO₂ and explain why each C=O double bond contains two shared pairs.
Give me a hint
Count the bonding pairs between carbon and each oxygen.
Compare my reasoning
- A covalent bond is attraction between both nuclei and shared electrons.
- A double bond contains two shared pairs.
- Each oxygen also has lone pairs in its Lewis representation.
Each C=O double bond contains two shared electron pairs attracted to the carbon and oxygen nuclei; CO₂ is represented as O=C=O with two lone pairs on each oxygen.
Look for these in your work
- Covalent definition applied
- Two shared pairs per double bond
- Oxygen lone pairs included
Explain how NH₃ and HCl form NH₄⁺ and describe the origin of the new bond.
Give me a hint
Identify the electron pair available on nitrogen and the accepting species.
Compare my reasoning
- Nitrogen in NH₃ has a lone pair.
- HCl supplies H⁺.
- Nitrogen donates its pair to H⁺, producing a shared pair in NH₄⁺.
NH₃ + H⁺ → NH₄⁺. Nitrogen donates both electrons in the new N–H shared pair, so the bond is coordinate on formation.
Look for these in your work
- Lone-pair donor named
- H⁺ acceptor identified
- Coordinate origin explained
A learner says sulfur cannot be shown with more than eight outer electrons and so SF₆ is impossible. Correct the model statement.
Give me a hint
Use the syllabus examples for expanded octets in Period 3.
Compare my reasoning
- Sulfur is a Period 3 element.
- The syllabus includes SF₆ as an expanded-octet example.
- Do not apply a strict four-pair limit to this specified representation.
The syllabus allows Period 3 atoms to expand their octet in examples including SF₆. A strict four-pair limit would misrepresent this specified compound.
Look for these in your work
- Period 3 identified
- SF₆ example used
- Model limit corrected
Common mistakes
- Defining a covalent bond as attraction between atoms only
- Showing lone pairs as bonds
- Saying the coordinate pair remains chemically different after formation
- Forcing SO₂, PCl₅ or SF₆ into a strict four-pair limit
What can you explain now?
What is shared in a covalent bond?
Compare with the explanation
A pair of electrons attracted to both nuclei
The shared pair is attracted by both bonded nuclei.
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Make it stick
How does the coordinate-bond electron source differ from an ordinary covalent pair, and what changes after formation?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.