Stereoisomers are defined as?

• A. Different molecular formula
• B. Different connectivity of atoms
• C. Same formula and same connectivity
• D. Same formula and connectivity, but different arrangement in space

Answer: (D)

Stereoisomerism arises when molecules have the same atoms and the same connections between them, but the atoms are arranged differently in three-dimensional space. This difference in spatial arrangement creates distinct forms that cannot be interconverted by simply rotating bonds, giving each form unique properties.

The statement that matches this is: same formula and connectivity, but different arrangement in space. That captures the idea that the two structures are built from the same components in the same order, yet their three-dimensional shapes differ.

The other descriptions point to different ideas. A different molecular formula means a different compound entirely. Different connectivity refers to structural (constitutional) isomers, where the bonding pattern is not the same. Identical connectivity and formula would imply the same molecule, not a separate stereoisomer. Examples include cis-trans isomers around a double bond or enantiomers around a chiral center, which differ only in spatial arrangement.