1.7.2Principles of Chemistry
Covalent Bonds- Dot and Cross Diagrams — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 43 of 50
Part 1 of 5Drawing Rules
Each covalent bond in a dot-and-cross diagram represents one shared pair of electrons.
Outer shell only
- Show only outer-shell electrons, with dots for one atom and crosses for the other atom in each shared pair.
Single bond
- One shared pair of electrons, which is drawn as a single line in a structural formula, as in H–H.
Double bond
- Two shared pairs, four electrons in total, as in oxygen, O=O, and carbon dioxide, O=C=O.
Triple bond
- Three shared pairs, six electrons in total, as in nitrogen, N≡N.
Lone pairs
- Electrons not used in bonding are drawn as pairs on the atom that holds them, as in water and ammonia.
Part 2 of 5Common Molecules
Counting bonds gives the number of shared pairs for each atom.
1
Oxygen has six outer electrons and needs two more, so it forms two covalent bonds with two hydrogen atoms in water.
2
Each hydrogen atom shares its single electron with one unpaired electron on the oxygen atom, forming an H–O bond.
3
The oxygen atom keeps two lone pairs, which are shown as dot pairs on the oxygen atom only.
4
Carbon has four outer electrons, so it forms four single bonds in methane and has no lone pairs left over.
| Molecule | Formula | Bonds and lone pairs |
|---|---|---|
| Hydrogen | H₂ | One single bond |
| Chlorine | Cl₂ | One single bond |
| Oxygen | O₂ | One double bond |
| Nitrogen | N₂ | One triple bond |
| Hydrogen chloride | HCl | One single bond |
| Water | H₂O | Two single bonds; two lone pairs on O |
| Ammonia | NH₃ | Three single bonds; one lone pair on N |
| Methane | CH₄ | Four single bonds; no lone pairs |
| Carbon dioxide | CO₂ | Two double bonds; no lone pairs |
Part 3 of 5Reading a Structure
Reading a Structure
Counting bonds
- Each line in a structural formula stands for one shared pair, so counting lines gives the number of covalent bonds.
Atom checks
- Each atom should have eight outer electrons in its diagram, except hydrogen, which has two.
Molecular formula
- Writing the formula from the diagram means counting every atom in one molecule, for example H₂O for water.
Part 4 of 5Worked Example
Question Draw a dot-and-cross diagram for ammonia, NH₃, and state how many lone pairs it has.
- Nitrogen has five outer electrons and needs three more to complete its outer shell.
- Each of the three hydrogen atoms shares one electron with nitrogen, forming three N–H bonds.
- Nitrogen keeps its two remaining outer electrons as one lone pair.
- Nitrogen therefore shows eight outer electrons in total, with three bonding pairs and one lone pair.
Answer One lone pair on nitrogen and three bonding pairs.
Part 5 of 5Exam Checklist & Tips
1.7.2 – Key Points for the Exam
- Dot-and-cross diagram A diagram showing the outer electrons of each atom in a molecule, with dots and crosses to show their origin.
- Bonding pair A shared pair of electrons forming a covalent bond.
- Lone pair A pair of outer electrons on one atom that is not used in bonding.
- Double bond Two shared pairs of electrons between the same two atoms.
- Triple bond Three shared pairs of electrons between the same two atoms.
Common Mistakes
- Counting a double bond as a single bond; a double bond contains two shared pairs of electrons.
- Leaving out the lone pairs on water and ammonia, which are needed to show all outer electrons.
- Giving oxygen three bonds in water instead of two, which breaks the rule for oxygen's valency.
- Drawing hydrogen with more than two electrons in its outer shell.
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