polyvalent ion vs monovalent ion

What's the difference between polyvalent ion and monovalent ion? Does mono and ploy refers to the number of electron shells? Why does monovalent ions yield a thick DDL?

Mono and polyvalent refers to the charge on the cation. For example K⁺ and Na⁺ are monovalent, while Ca²⁺ and Mg²⁺ are polyvalent.

In soils, recall that most colloids have a net negative charge, thus they attract cations. The number of cations needed to "counter" the net negative charge on soil colloids depends on their valence; e.g. two K⁺ cations versus one Ca²⁺. The thickness of the DDL depends on both the number of cations attracted toward the colloid and their hydrated radius. Again, it will take more monovalent cations to balance the charge than polyvalent, leading to a thicker DDL. Na⁺ is particularly interesting as it is monovalent AND has a large hydrated radius, so if there is a lot of Na in the soil solution, then the DDL will be thick, and particles will tend to disperse.