What is the main characteristic of stereoisomers?

Stereoisomers are distinguished primarily by their three-dimensional spatial arrangement of atoms, despite having the same molecular formula. While they share the same connectivity of atoms (hence the same structural formula), their differences arise in how those atoms are oriented in space. This spatial distinction can lead to varying physical and chemical properties, such as boiling points, melting points, and reactivity.

The correct answer emphasizes that even though stereoisomers are identical in terms of their molecular formula (the types and numbers of atoms present), the variation in their 3D arrangement is what defines their stereoisomeric nature. This characteristic is crucial in fields like pharmacology, where the efficacy and effects of compounds can change dramatically between stereoisomers, despite them being composed of the same atoms.