Draw dot-and-cross diagrams for bonding
Represent ionic, covalent and coordinate bonding, including named structures with expanded octets and an odd electron.
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 does a dot-and-cross mark identify?
What you will learn
- Use dot-and-cross diagrams for ionic, covalent and coordinate bonds
- Represent specified molecules and ions, including expanded-octet and odd-electron species
A dot-and-cross diagram tracks electron origin
Choose a dot for electrons from one atom and a cross for electrons from another. A shared pair between nuclei represents a covalent bond; a coordinate pair can be shown with both marks from the donor atom. The marks show origin, not different physical electron properties.
For ionic compounds, show electron transfer, bracket the resulting ions and label their charges. Include outer-shell electrons relevant to the formula: Na⁺/Cl⁻, Mg²⁺/O²⁻ and Ca²⁺/F⁻ are the syllabus examples for ionic bonding.
Follow the named electron count, including specified exceptions
Use diagrams for the molecules listed in 3.4 and 3.5: H₂, O₂, N₂, Cl₂, HCl, CO₂, NH₃, CH₄, C₂H₆, C₂H₄, SO₂, PCl₅, SF₆, NH₄⁺, Al₂Cl₆, BF₃, PF₅ and H₂O. In dot-and-cross work, every covalent pair must be counted once, and lone pairs remain visible.
The syllabus allows specified diagrams with expanded octets, such as PCl₅ and SF₆, and species with an odd number of electrons. Do not add or remove electrons to make every drawing fit an octet. Check total valence-electron count and ion charge after drawing.
Pause and explain
How should the donated pair in NH₄⁺ formation be represented initially?
Worked example
Describe how a dot-and-cross diagram represents NH₄⁺ formation from NH₃ and H⁺.
Show the worked solution
- Draw the three N–H shared pairs already present in NH₃ and its lone pair.
- Show both electrons of that lone pair forming the new N–H shared pair with H⁺.
- Bracket the resulting ion and label +; check the total valence-electron count.
Answer The fourth N–H pair is shown with both marks from nitrogen to record coordinate donation; bracket the whole ion as NH₄⁺. After formation it is a shared pair.
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 dot-and-cross representations for NaCl and CaF₂, including the ion charges.
Give me a hint
Show electron transfer before placing brackets and charges.
Compare my reasoning
- Na transfers one electron to Cl, forming Na⁺ and Cl⁻.
- Ca transfers one electron to each of two F atoms.
- Show [Ca]²⁺ and two [F]⁻ ions with full outer shells.
NaCl is shown as [Na]⁺[Cl]⁻. CaF₂ is shown as [Ca]²⁺ with two [F]⁻ ions; each transferred electron is marked as coming from the metal.
Look for these in your work
- Transfer count for NaCl correct
- Two transfers for CaF₂
- Brackets and charges correct
In a dot-and-cross diagram for CO₂, what does each C=O bond contain and what remains on each oxygen?
Give me a hint
Count shared pairs and non-bonding pairs separately.
Compare my reasoning
- Each C=O double bond has two shared electron pairs.
- Each oxygen has two lone pairs in the usual Lewis diagram.
- Check that carbon and both oxygen atoms have the correct total valence electrons.
O=C=O has two shared pairs in each C=O bond and two lone pairs on each oxygen.
Look for these in your work
- Two shared pairs per double bond
- Two lone pairs per oxygen
- Electron count checked
Explain how a diagram can represent the expanded octet in SF₆ and the odd electron in a radical such as ClO₂.
Give me a hint
Use the stated total electrons; do not force an octet where the syllabus allows an exception.
Compare my reasoning
- Count all valence electrons for the neutral or charged species.
- Show the six S–F bonds in the expanded-octet representation.
- For the odd-electron species, leave the unpaired electron visible rather than inventing a pair.
SF₆ may be drawn with an expanded octet around sulfur. In the specified odd-electron species, retain the unpaired electron and verify the total electron count instead of forcing an octet.
Look for these in your work
- Expanded-octet exception applied
- Six bonds represented
- Odd electron retained and counted
Common mistakes
- Changing a formula subscript to make an electron diagram fit
- Omitting ion brackets or charge
- Drawing an arrow but no resulting shared pair
- Forcing expanded-octet or odd-electron examples into a complete octet
What can you explain now?
What does a dot-and-cross mark identify?
Compare with the explanation
Which atom supplied an outer electron in the diagram
The symbols distinguish electron origin for bookkeeping.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What should be checked after a dot-and-cross drawing: total electrons, shared pairs, lone pairs, ion brackets, or charge?
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.