Carbon bonding - model set

[ecoli]

I got a chemistry model building set (balls and sticks), and it came with carbon atoms that have 4 holes (which makes sense) but also with carbons that have 5 holes. I never knew that you can have carbon molecules that can make 5 bonds. When does it do this?

[akcapr]

it doesnt

[ecoli]

it doesnt

 

Then why did they give me six carbon atoms with 5 holes??

[woelen]

How are these holes oriented? I assume that the carbon atoms with 4 holes have all holes oriented towards the end of a tetrahedron.

 

For double-bonded molecules, like H2C=CH2, the 3D structure is not tetrahedral, but it is flat.

 

With these 5-hole carbons, you probably can model molecules like these. Also, can you model molecules like HC≡CH, with a triple bond?