In trimethylamine, ${\left({\mathrm{CH}}_3\right)}_3\mathrm{N}:$ three ${\mathrm{CH}}_3$ groups are attached to the nitrogen by $\mathrm{C}-\mathrm{N}$ bonds. The repulsion of the ${\mathrm{CH}}_3$ groups is slightly greater than the repulsion of the $\mathrm{H}$ atoms in ammonia. Which one of these bond angles for the $\mathrm{C}-\mathrm{N}-\mathrm{C}$ bond in trimethylamine, is correct?

Answer the following, giving a full explanation in terms of metallic bonding:

    The thermal conductivity of stainless steel is 82 W m^-1 K^-1 . The thermal conductivity of  copper  is 400 W m^-1 K^-1. Why do some stainless steel saucepans have a copper base? 

 Answer the following, giving a full explanation in terms of metallic bonding:

     Why does aluminium conduct electricity better than sodium? 

Is the following molecule polar or non-polar? Give a reason for your answer. (Electronegativity values: F = 4.0, Cl = 3.0, Br= 2.8, S =2.5, C = 2.5, H=2.1) 
 

                               chlorine, Cl₂ 

Is the following molecule polar or non-polar? Give a reason for your answer. (Electronegativity values: F = 4.0, Cl = 3.0, Br= 2.8, S =2.5, C = 2.5, H=2.1) 
 

                              hydrogen fluoride, HF 

 Is the following molecule polar or non-polar? Give a reason for your answer. (Electronegativity values: F = 4.0, Cl = 3.0, Br= 2.8, S =2.5, C = 2.5, H=2.1) 
 

                              The V-shaped molecule, sulfur dichloride, SCI₂

Is the following molecule polar or non-polar? Give a reason for your answer. (Electronegativity values: F = 4.0, Cl = 3.0, Br= 2.8, S =2.5, C = 2.5, H=2.1) 
 

                   The tetrahedral molecule, chloromethane, CH₃Cl            

Is the following molecule polar or non-polar? Give a reason for your answer. (Electronegativity values: F = 4.0, Cl = 3.0, Br= 2.8, S =2.5, C = 2.5, H=2.1) 
 

                   The tetrahedral molecule, tetrabromomethane, CBr₄