[navalava]

My workbook asks for an explanation for this question:
Why don't terminal alkenes (alk-1-enes), such as ethene/propene exist as stereoisomers?
The answer is supposed to be "One of the carbons in the alkene linkage has two like atoms (groups) attached to it."
I don't get what that has to do with being a stereoisomer...can't the like atoms be placed on opposite sides to form trans isomers?

But later on it asks me to give the formula of trans-1,2-dichloropropene...aren't cis and trans isomers examples of stereoisomers? Wouldn't that contradict their previous question that ethene and propene don't exist as stereoisomers?

I would really appreciate some help on this. Thanks!

[Polytriazole]

In order to get stereoisomers across alkenes, each carbon in the double bond has to have substituents that are not the same.  That way, the dissimilar substituents can either be cis (on the same side of the bond) or trans (on opposite sides of the bond).  In terminal alkenes, the terminal carbon is bonded to two hydrogen atoms.  Because these substituents are identical, it doesn't matter what side of the bond a particular hydrogen is on.  If you swapped out one of the terminal hydrogens for a deuterium, then you could get cis/trans isomerism.

As for your second question:  you're being given the name of a propene that has a chlorine on either side of the double bond (you should do a quick review of IUPAC naming rules for alkenes).  Draw the structure and then try to look for the cis/trans isomerism. 

[orgopete]

My workbook asks for an explanation for this question:
Why don't terminal alkenes (alk-1-enes), such as ethene/propene exist as stereoisomers?
…But later on it asks me to give the formula of trans-1,2-dichloropropene...aren't cis and trans isomers examples of stereoisomers? Wouldn't that contradict their previous question that ethene and propene don't exist as stereoisomers?

The question assumed students knew what terminal alkenes were as a class. It would have been okay if it simply asked why ethene and propene don't exist as stereoisomers. Then it would have avoided the naming ambiguity of propene and 1,2-dichloropropene regarding stereoisomers.

This is a subtle point in the naming being raised in this question. I would not call 1,2-dichloropropene a terminal alkene, but I can also understand the posters question in this regard.  (Perhaps a follow up question could have been "Why isn't 1,2-dichloropropene a terminal alkene?")