But-1-ene and cyclobutane are................isomers.

Answer

To determine the type of isomers that but-1-ene and cyclobutane represent, we can follow these steps:

### Step 1: Identify the Structures
- **But-1-ene** is an alkene with the formula C4H8. Its structure can be represented as:
\[ \text{CH}_2=CH-CH_2-CH_3 \]
- **Cyclobutane** is a cyclic alkane with the same molecular formula C4H8. Its structure can be represented as:
\[ \text{A square or cyclic structure with 4 carbon atoms} \]

### Step 2: Compare the Structures
- But-1-ene has a linear structure with a double bond, while cyclobutane has a cyclic structure. The connectivity of the atoms is different in both compounds.

### Step 3: Classify the Isomers
- Since but-1-ene and cyclobutane have the same molecular formula (C4H8) but different structural arrangements, they are classified as **structural isomers**. Structural isomers have the same molecular formula but differ in the connectivity of atoms.

### Conclusion
But-1-ene and cyclobutane are **structural isomers**.

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