[jjc]

Ksp of AgCl is 1.8x10^-10,
Ksp of AgBr is 3.3x10^-13,
Ksp of AgI is 1.5x10^-16

I would like to ask how come the solubility product constant decreases as size of anion increases

thanks

[Rabn]

There are a couple ways you can reason through this:

1) The electronegativity difference between the atoms decreases as the anion's size increases so the bond becomes less ionic and more covalent.

2) The valence electron shell of the larger anion is much more "plastic", meaning it is more flexible, because the electrons aren't held as tightly to the atom due to blocking effects from the core electrons. This causes the bond to be stronger since the valence electrons are easier to share.

3) The two previous reasonings are both sides of the same coin, it all comes down to energy.  Silver has a 5s orbital which is half full(5s¹), the orbital will be stable if it loses an electron or gains an electron.  The larger the anion, the easier it is for it to provide an electron to fill the 5s orbital; using reason 2. The larger the anion, the lower it's electronegativity; this means that it has less of a pull on the electron that silver can provide and so the bond becomes more covalent; using reason 1.  As you can see each one is a description each atoms ability to fill it's outer shell with number 2 describing Ag filling its outer shell and number 1 describing the anion filling its outer shell.

Hope this helped.