Hard and soft acids and bases (HSAB)

The HSAB (Hard and Soft Acids and Bases) theory helps predict how chemical species interact. "Hard" acids and bases are small, highly charged, and less polarizable—they prefer to bond with similar partners. "Soft" acids and bases are larger, less charged, and more polarizable—they tend to pair up with other soft counterparts. This concept aids chemists in understanding reaction preferences, stability, and designing materials or drugs by matching compatible acid-base types for more reliable interactions.