A LevelChemistryCIETopic Questions3. Chemical Bonding3.7 Dot-&-Cross Diagrams

Syllabus Edition

First teaching 2023

First exams 2025

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Dot-&-Cross Diagrams (CIE A Level Chemistry)

Topic Questions

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1a2 marks

This question is about various polyatomic ions.

The nitrate(V) ion, NO3-, is a polyatomic ion, bonded by covalent bonds. 
The three oxygen atoms are bonded by one single covalent bond, one double covalent bond and one coordinate bond.

Draw the dot-and-cross diagram for NO3-. 

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1b2 marks

An ionic compound has the empirical formula H4N2O3. 

Suggest the formulae of the ions present in this compound.

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1c2 marks

Cyanide is a fast-acting chemical, which can be found in various forms and can have toxic effects on the body.

Draw the dot-and-cross diagram for a CN- ion. 

Show the outer electrons only.

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1d4 marks

i)

Compare the average bond enthalpy in the cyanide ion to the average bond enthalpy of the C-N bond in methylamine, CH3NH2. Explain your answer.
[2]
ii)
Explain why the C-N bond length in the cyanide ion is shorter than in methylamine.

[2]

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1a3 marks

Dot and cross diagrams are used to show the arrangement of outer electrons in covalently bonded molecules.

Draw diagrams for the following molecules:

i)
Hydrogen cyanide.
 
hcn-dot-and-cross-blank
 
[1]
 
ii)
Carbon dioxide.
 
co2-dot-and-cross-blank
[1]
 
iii)
Boron trifluoride.
 
nwTWgR4K_bf3-dot-and-cross-blank
[1]

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1b2 marks

Using your answer to part (a), identify the species that is likely to form a coordinate bond. Explain your answer.

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1c2 marks
i)
Draw a dot-and-cross diagram for the product of boron trifluoride and ammonia.
 
[1]
 
ii)
Explain how the bond forms between the boron trifluoride and ammonia molecules.
 
[1]

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2a2 marks

Phosphorus tribromide and sulfur tetrafluoride are two colourless compounds which both react with water to form toxic products. 

Draw diagrams to show the arrangement of outer electrons in both compounds.

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2b2 marks

Predict the shapes of the phosphorus tribromide molecule.

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2c2 marks

Explain why both phosphorus tribromide and sulfur tetrafluoride are polar.

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3a1 mark

Ammonia, NH3, is a key chemical in manufacturing certain fertilisers and cleaning products.

Write an equation to show the formation of an ammonium ion from ammonia.

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3b1 mark

Predict the shapes of an ammonia molecule and an ammonium ion.

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3c2 marks

Draw a dot-and-cross diagram to show the bonding in the ammonium ion.

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