2.5 The Shapes of Simple Molecules & Ions

Tetrafluorothene is best known for being the monomer that forms poly(tetrafluoroethene), PTFE, otherwise known by the trade name Teflon and used in non-stick cookware. Tetrafluorohydrazine, however, is a toxic and irritant gas with few significant commercial uses.

Tetrafluoroethene, C₂F₄, and tetrafluorohydrazine, N₂F₄, are fluorides of adjacent elements in the Periodic Table.

Draw the dot-and-cross diagrams for C₂F₄ and N₂F₄ showing all valance electrons.

Predict and explain the F-C-F bond angle in tetrafluoroethene and the F-N-F bond angle in tetrafluorohydrazine.

Tetrafluorohydrazine is a polar molecule but tetrafluoroethene is not.

Explain the difference in molecular polarity.

Methanol is often used as a solvent, commonly in the production of inks and dyes. However, it is also highly soluble itself in water.

Explain why methanol is soluble in water.

Methanol, ethanol and propan-1-ol are all primary alcohols.
Describe and explain the trend in their melting points shown below. 

These longer primary alcohols have the following melting points: 

Describe and explain this trend.

Predict, with a reason, which of ethanol or ethane-1,2-diol will have the higher melting point?

Alkanes are naturally occurring hydrocarbons present as a mixture in crude oil. In order for them to be useful the mixture needs to be separated into fractions, which is done using a temperature gradient as they have varying boiling points.

C₂H₆, C₄H₁₀ and C₃H₈ are all examples of alkanes. 

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Predict, with a reason, whether alkanes are soluble in either water or propanone.

Pentane can exist as isomers, including pentane, CH₃CH₂CH₂CH₂CH₃ and 2,2-dimethylpropane, CH₃C(CH₃)₂CH₃.

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There are two isomers possible with the molecular formula C₂H₆O. One is an alcohol and one is an ether, which has a C-O-C functional group.

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Halogens can form covalent compounds between themselves, called interhalogen compounds, as well as with other elements.

One interhalogen compound is IF₅. 

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Iodine can also form the triiodide ion, I₃^-. 

Halogens can also form ionic compounds.
Sodium chloride and iodine are both solids. Sodium chloride does not melt until it reaches a temperature of 1074 K, yet iodine sublimes when heated gently, giving off purple vapours. 

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Sodium chloride will conduct electricity when molten, not solid, but iodine is a very poor conductor of electricity in all states.

Explain the differences in electrical conductivity of sodium chloride and iodine.

The electronegativities of some elements are shown in the table below. 

Using the table, explain why the bonding in ammonia (NH₃) is covalent but the bonding in lithium chloride (LiCl) is ionic.

Sketch the shape of a methane (CH₄) molecule. In your sketch indicate any lone pairs of electrons, name the shape of the molecule and suggest the bond angle. 

Explain why tetrachloromethane, CCl₄, is a non polar molecule, whilst CH₃Cl, is polar.

Chlorine (Cl) reacts with boron (B) to form boron trichloride (BCl₃).

Molecules and separate atoms (such as noble gases) are attracted to one another by intermolecular forces. 

The boiling point of tetrachloromethane (CCl₄) is 77 ^oC whereas the boiling point of water (H₂O) is 100 ^oC. 

Deduce whether each of the three molecules oxygen difluoride, OF₂, phosphorus trifluoride, PF₃, and boron trichloride, BCl₃, are polar or non-polar.

Give a reason in each case.

Boron trichloride can react with a chloride ion (Cl^-) to form a boron tetrachloride ion, (BCl₄^-)

The structures of two hydrocarbons are shown below.

The boiling point of hydrocarbon B is higher than that of hydrocarbon A. ExplaIn this difference in boiling points.

Three examples of boron compounds are B(OH)₃, BF₄^- and BH₃. State which of these compounds is not planar and explain your answer. 

State the compound in part (c) that has the highest boiling point. Explain your answer.

Solids have higher densities than liquids. An exception to this is ice. The image below shows the three-dimensional network of bonds in ice.

Using intermolecular forces, explain why ice has a density which is lower than liquid water. 

Both water, H₂O, and phosphine, PH₃, have the same molecular mass of 18.0 however water has a much higher melting point, 0 ^oC than phosphine, -133 ^oC

A PH₃ molecule can react with a hydrogen ion (H⁺) to form a PH₄⁺ ion.

Phosphoric acid (H₃PO₄) is another phosphorus compound that is often used in making fertilisers and detergents. Unlike phosphine (PH₃), phosphoric acid can form hydrogen bonds. The shape of phosphoric acid is given below.