The polarizability of an acid or base plays a role in its reactivity.  Hard acids and bases are small, compact, and non-polarizable.

  Soft acids and bases are larger, with a more diffuse distribution of electrons.

In addition to their intrinsic strength,

  Hard acids react preferentially with hard bases, and soft acids react preferentially with soft bases.

Compound    solubility product

     AgF  205

    AgCl  1.8 x 10-10

    AgBr  5.2 x 10-13

     AgI  8.3 x 10-17

AgX(s)  +  H2O(l)  ↔  Ag+(aq)  + X-(aq)

There have been many attempts to categorize various metal ions and anions to predict reactivity, solubility, etc.

  R.G. Pearson (1963) categorized acids and bases as either hard or soft (using Kf values). 

  Hard acids bond in the order:   F->Cl->Br->I-

  Soft acids bond in the order:  I- >Br- >Cl- > F-

The polarizability of an acid or base plays a role in its reactivity.  Hard acids and bases are small, compact, and non-polarizable.

  Soft acids and bases are larger, with a more diffuse distribution of electrons.

In addition to their intrinsic strength,

  Hard acids react preferentially with hard bases, and soft acids react preferentially with soft bases.

Compound    solubility product

     AgF  205

    AgCl  1.8 x 10-10

    AgBr  5.2 x 10-13

     AgI  8.3 x 10-17

AgX(s)  +  H2O(l)  ↔  Ag+(aq)  + X-(aq)

There have been many attempts to categorize various metal ions and anions to predict reactivity, solubility, etc.

  R.G. Pearson (1963) categorized acids and bases as either hard or soft (using Kf values). 

  Hard acids bond in the order:   F->Cl->Br->I-

  Soft acids bond in the order:  I- >Br- >Cl- > F-