AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to: draw dot and cross diagrams for the molecules…
Full specification wording
Covalent bonding: Students should be able to: draw dot and cross diagrams for the molecules of hydrogen, chlorine, oxygen, nitrogen, hydrogen chloride, water, ammonia and methane represent the covalent bonds in small molecules, in the repeating units of polymers and in part of giant covalent structures, using a line to represent a single bond describe the limitations of using dot and cross, ball and stick, two and three-dimensional diagrams to represent molecules or giant structures deduce the molecular formula of a substance from a given model or diagram in these forms showing the atoms and bonds in the molecule.
You need to draw dot and cross diagrams for hydrogen, chlorine, oxygen, nitrogen, hydrogen chloride, water, ammonia and methane, showing outer-shell electrons and shared pairs.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 8 of 8
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
You need to draw dot and cross diagrams for hydrogen, chlorine, oxygen, nitrogen, hydrogen chloride, water, ammonia and methane, showing outer-shell electrons and shared pairs. You also represent covalent bonds with lines in small molecules, polymer repeating units and parts of giant covalent structures. Each model has limits: dot and cross diagrams show electrons but not shape; ball and stick models show three-dimensional shape but not electron pairs; two-dimensional diagrams can hide the true shape. From any model or diagram, count the atoms of each element to deduce the molecular formula, such as H₂O or CH₄.
What earns the marks
- Dot and cross diagrams must show outer-shell electrons only, with shared pairs in the overlap between atoms.
- Hydrogen, chlorine, oxygen and nitrogen are diatomic; oxygen and nitrogen contain double and triple bonds respectively.
- Water, ammonia and methane have different numbers of bonds per central atom: two, three and four respectively.
- Line representations use one line per single bond and can be used for small molecules, polymer repeating units and parts of giant covalent structures.
- Limitations include lack of shape information in dot and cross diagrams and lack of electron detail in ball and stick models.
- Molecular formula is deduced by counting the atoms of each element shown in the model or diagram.
Common mistakes and how to put them right
Showing all electrons rather than only outer-shell electrons in a dot and cross diagram.
Put it right by drawing all outer-shell electrons, including both shared bonding pairs and non-bonding lone pairs.
Drawing oxygen with a single bond.
Put it right by showing a double bond, because oxygen atoms share two pairs of electrons.
Giving the molecular formula as the empirical formula or missing subscripts.
Put it right by counting every atom shown, for example C₂H₆ rather than CH₃.
Exam tips
- Check the group number of each atom to work out how many covalent bonds it forms before drawing.
- When deducing a molecular formula, count atoms systematically and use subscript numbers for more than one atom.
- For limitations, name one specific feature each model cannot show rather than saying it is simply inaccurate.
Try the lesson
The start of the Covalent bonding: Students should be able to: draw dot and cross diagrams for the molecules of hydrogen, chlorine, oxygen, nitrogen, hydrogen chloride, water, ammonia and methane represent the covalent bonds in small molecules, in the repeating units of polymers and in part of giant covalent structures, using a line to represent a single bond describe the limitations of using dot and cross, ball and stick, two and three-dimensional diagrams to represent molecules or giant structures deduce the molecular formula of a substance from a given model or diagram in these forms showing the atoms and bonds in the molecule. lesson, made from checked teaching notes for this specification point.
Have a go first
In a dot and cross diagram of a water molecule, what does each dot or cross represent?
Listen
You need to draw dot and cross diagrams for hydrogen, chlorine, oxygen, nitrogen, hydrogen chloride, water, ammonia and methane, showing outer-shell electrons and shared pairs. You also represent covalent bonds with lines in small molecules, polymer repeating units and parts of giant covalent structures. Each model has limits: dot and cross diagrams show electrons but not shape; ball and stick models show three-dimensional shape but not electron pairs; two-dimensional diagrams can hide the true shape. From any model or diagram, count the atoms of each element to deduce the molecular formula, such as H₂O or CH₄. A strong answer earns marks for these points. Dot and cross diagrams must show outer-shell electrons only, with shared pairs in the overlap between atoms. Hydrogen, chlorine, oxygen and nitrogen are diatomic; oxygen and nitrogen contain double and triple bonds respectively. Water, ammonia and methane have different numbers of bonds per central atom: two, three and four respectively. Line representations use one line per single bond and can be used for small molecules, polymer repeating units and parts of giant covalent structures. A common slip: Showing all electrons rather than only outer-shell electrons in a dot and cross diagram. Instead, drawing all outer-shell electrons, including both shared bonding pairs and non-bonding lone pairs. Exam tip: Check the group number of each atom to work out how many covalent bonds it forms before drawing.
See it
Key words
Say what each word means before you turn the card.
Dot and cross diagrams
Try answering aloud or in your head before turning the card.
Line-bond representations
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Model limitations
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See every definition
- Dot and cross diagrams
- Diagrams that show outer-shell electrons only, with shared pairs in the overlap between atoms.
- Line-bond representations
- Representations that use one line per single bond, for small molecules, polymer repeating units and parts of giant covalent structures.
- Model limitations
- Each model has limits: dot and cross diagrams show electrons but not shape, ball and stick models show three-dimensional shape but not electron pairs, and two-dimensional diagrams can hide the true shape.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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Test your understanding of Covalent bonding: Students should be able to: draw dot and cross diagrams for the molecules of hydrogen, chlorine, oxygen, nitrogen, hydrogen chloride, water, ammonia and methane represent the covalent bonds in small molecules, in the repeating units of polymers and in part of giant covalent structures, using a line to represent a single bond describe the limitations of using dot and cross, ball and stick, two and three-dimensional diagrams to represent molecules or giant structures deduce the molecular formula of a substance from a given model or diagram in these forms showing the atoms and bonds in the molecule.
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