The number of structural isomers possible for the molecule C₃H₉N is

[Step 1]:
The possible structural isomers of C₃H₉N are

1) ${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{NH}}_2$ (1° amine)

2) ${\mathrm{CH}}_3-\underset{\underset{{\mathrm{NH}}_2}{|}}{\mathrm{C}}\mathrm{H}-{\mathrm{CH}}_3$ (1°amine)

3) ${\mathrm{CH}}_3-\mathrm{NH}-{\mathrm{CH}}_2-{\mathrm{CH}}_3$ (2° amine)

4) $C{H}_3-\underset{\underset{C{H}_3}{|}}{N}-C{H}_3$ (3° amine)