What is the structure of (Z)-1-chloro-1-fluoro-1-butene?

#Z# means when you compare two substituents across the double bond, when looking at each carbon of the double bond, you use the substituent that is higher in priority relative to the one attached to that same carbon. When you compare those substituents, #Z# means they are on the same side.

(I remember it as "zee same side". :P Also, the Z stands for Zusammen, the German word for together)

The 1-chloro-1-fluoro says that chlorine and fluorine are both on carbon-1. These two substituents will be compared for priority to determine whether I drew #Z# or #E#.

The 1-butene further confirms that the double bond is across carbon-1 and carbon-2, and that the chlorine and fluorine are attached directly to the double bond on carbon-1.

To confirm this structure, what are the priorities of the substituents attached across the double bond? In this case, they are based on atomic number, because all the attached elements here are different.

#"Cl" > "F" > "CH"_2"CH"_3 > "H"#

Since chlorine is higher priority than fluorine, and the ethyl group is higher priority than hydrogen, the ethyl group (higher-priority on carbon-2) should indeed be on the same side as the chlorine (higher-priority on carbon-1).