4.2 Resonance, Shapes & Giant Structures

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

Draw the Lewis (electron dot) structures 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.

Draw the Lewis (electron dot) structure of the carbonate ion, CO₃^2-.

Deduce the number of possible resonance structures for the carbonate ion, CO₃^2-, and draw two of them.

Discuss how the bonding in the carbonate ion, CO₃^2-, evidences the presence of the resonance structures.

Organic molecules can also show resonance. The methanoate ion, HCOO^-, shows similar resonance forms to the carbonate ion, CO₃^2-.

The corresponding organic acid, methanoic acid, also has resonance structures.

Draw another resonance structure of methanoic acid.

Some of the physical and structural properties of diamond and graphite are shown below:

Suggest why the melting point of graphite is higher than that of diamond, using the information in the table.

Predict the bond order in both diamond and graphite.

Graphene has the structure of a single layer of graphite. 

Suggest, giving a reason, the electron mobility of graphene compared to graphite.

Graphite is a layered giant structure, containing London dispersion forces between the layers, whereas diamond has covalent bonds across all planes.

Describe and explain, based on structure and bonding, the differences expected when each of graphite and diamond are moved across a paper surface.

In 1996 the Nobel prize in Chemistry was awarded for the discovery of a new carbon allotrope, known as fullerenes. 

Outline the structure of buckminsterfullerene.

Like carbon dioxide, graphite is also a covalent substance, but it is a solid at room  temperature. Graphite has a melting point of around 3600 ^oC.           

Describe the structure and bonding of graphite and explain why it has such a high melting point.

Graphite is made purely of carbon, a non-metal, yet it conducts electricity.
Diamond, which is also made purely of carbon, cannot conduct electricity. 

i)         Explain this difference in electrical conductivity between graphite and diamond.            

ii)        Give one other difference in the properties of graphite and diamond.

Graphite is soft and so is used as a lubricant, whereas diamond is hard and so is used in many cutting tools. Both are giant covalent structures.

Explain this difference with reference to structure and bonding.

The Valence Shell Electron Pair Repulsion Theory (VSEPR) is used to predict the shapes of many chemical molecules.

Describe the main features of the VSEPR theory for predicting shapes of molecules.

State and explain the bond angle F-O-F in OF₂.

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.

Predict and explain the shapes and bond angles of the following molecules:

i)        BF₃ 

ii)       NBr₃

Ethene, C₂H₄, and hydrazine, N₂H₄, are hydrides of adjacent elements in the periodic table. 

State and explain the H一C一H bond angle in ethene and the H一N一H bond angle in hydrazine.

Hydrazine can be oxidised to form diimide, which is a useful compound used in organic synthesis. 

Deduce the molecular geometry of diimide, N₂H₂, and estimate its H–N–N bond angle.

Explain whether ethene and hydrazine are polar or non-polar.

Hydrazine forms a cation with an ethane-like structure called hydrazinediium,  N₂H₆²⁺.

Predict the value of the H–N–H bond angle in N₂H₆²⁺.

Draw the resonance structures for the following ions:

i)        Methanoate, HCOO^-.

ii)       Nitrate(III), NO₂^–.

Deduce the resonance structures of the carbonate ion, giving the shape and the oxygen-carbon-oxygen bond angle.

In December 2010, researchers in Sweden announced the synthesis of N,N–dinitronitramide, N(NO₂)₃. They speculated that this compound, more commonly called trinitramide, may have significant potential as an environmentally friendly rocket fuel oxidant. 

Deduce the N–N–N bond angle in trinitramide and explain your reasoning.

Predict, with an explanation, the polarity of the trinitramide molecule.