The number of structural isomers possible for ${C_4}{H_8}$ is.
1) $4$
2) $3$
3) $5$
4) $6$

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Hint: We know that the molecules with the same molecular formula but different molecular geometries are called isomers. There are a number of types of isomers. They are,
Conformational isomers.
Constitutional isomers.

Complete step by step answer:
We must know that an alkane with 4 carbon atoms would have the formula \[\;{C_4}{H_{10}}\] but the given hydrocarbon has two hydrogens less, so it must contain either a double bond or a ring. There are five possible isomers. They are But-1-ene, But-2-ene, 2-Methylpropane, Cyclobutane, and methyl cyclopropane.We can draw the all possible isomeric structures as,
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Therefore, the option 3 is correct.

Additional Information:
We know that if there is no superimposable mirror image and there is no plane of symmetry in the molecule then the molecule is called a chiral. This property is known as chirality and the compounds with the same molecular formula but differ in the arrangement of the atom are said to be isomers. The compounds which are mirror images but are not identical; to each other are called enantiomers.

Now we discuss how structural isomers, stereoisomers and conformational isomers differ from each other.
1.Structural isomer:
The molecules which have the same molecular formula but differ by the atoms or bonds are called a 2.structural isomer. It is also known as a constitutional isomer. There are two types of Structural isomer; they are (1) chain isomers and (2) ring isomers.
The isomers which differ by the orientation of atoms in space are called stereoisomers. There are two types of Stereoisomers; they are (1) Geometrical isomer and (2) optical isomer.
Conformational isomer:
4.The isomers that are different by their rotation around a single bond are called Conformational isomers.