Dot-&-Cross Diagrams — Structured Questions (Cambridge International AS Chemistry): Exam Questions

Draw dot-and-cross diagrams for the following molecules.

Show outer shell electrons only.

i) Hydrogen cyanide, HCN.

ii) Carbon dioxide, CO₂.

iii) Boron trifluoride, BF₃.

Extract:

Draw dot-and-cross diagrams for the following molecules.

Show outer shell electrons only.

i) Hydrogen cyanide, HCN.

ii) Carbon dioxide, CO2.

iii) Boron trifluoride, BF3.

Use your diagrams in (a) to deduce which of the three molecules is most likely to form a coordinate (dative covalent) bond. Explain your answer.

Extract:

The molecule from (a) most likely to form a coordinate (dative covalent) bond is:

• Boron trifluoride (BF3)

Explanation:

• Boron's outer shell has space for two more electrons

OR

Boron only has six outer shell electrons / boron has an incomplete outer shell

i) Draw a dot-and-cross diagram for the product of boron trifluoride and ammonia.

Show outer shell electrons only.

ii) Explain how the coordinate (dative covalent) bond forms between the boron trifluoride and ammonia molecules.

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.

State the shape of, and the Br-P-Br bond angle in, a molecule of PBr₃.

Extract:

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

Explain why molecules of PBr₃ and SF₄ are polar.

Construct an equation for the reaction of ammonia, NH₃, with a hydrogen ion.

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

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

Show outer shell electrons only.

Draw a dot-and-cross diagram for the nitrate(V) ion, NO₃^-.

Show outer shell electrons only.

An ionic compound has the empirical formula H₄N₂O₃.

Suggest the formulae of the ions present in this compound.

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

Show outer shell electrons only.

Compare the C≡N bond energy and bond length in the cyanide ion, CN^-, with the C-N bond energy and bond length in methylamine, CH₃NH₂.

Explain your answer.