Lewis structures of H₂(C)xCH₂ molecules I need to calculate the angles between CCC, CCH, HCH in the following molecules:


ethene ($\ce{H2CCH2}$), 
propa-1,2-diene ($\ce{H2CCCH2}$), and 
buta-1,2,3-triene ($\ce{H2CCCCH2}$). 


If I'm not mistaken, the Lewis structures of all of these molecules have the carbons in the middle, connected by double bonds to each other, and the hydrogen atoms connected to the carbons in the edge. For example:

H           H
 \         /
  C = C = C
 /         \
H           H


The carbons at the edge are $\mathrm{sp^2}$ and the middle carbon is always $\mathrm{sp}$ . 

This means that the angle CCC is always 180°. But how can I determine the angles CCH, HCH? 

In addition, how can I determine if the hydrogen atoms in the three molecules lie on the same plane?
What are the bond angles in a normal trigonal planar arrangement? Treat the center atom as in H C H as a trigonal planar arrangement with the middle (carbon) atom in the center with a third atom attached on the left over position (another carbon). What angle maximizes their separation?

Your other question about the plane of the molecule is a bit more interesting. The pi bonds between the carbons ensure that the molecule is not rotating in space. To maximize the distance between the terminal hydrogen atoms, one set should be aligned up and down, while the other two on the opposite side should be rotated 90&deg; in space, otherwise they would ellipse one another. In other words, one pair is horizontal while the opposite is vertical for example, depending how you want to imagine it.