Why do small anions have high hydration energy?

See, hydration of a ion indicates its relation with water molecule so you can call it bonding in general sense. read more

Becuase they have a high charge density or you should say they have the tendency to attract a large number of oppositely charged ions even if they are low in number . For example lithium ion. It has the highest hydration energy in group elements due to its high charge density. I hope this helps. read more

whereas the hydration energy has separate anion and cation terms. EH∝1r2++1r2−. [math]EH∝1r+2+1r−2.[/math] Generally the solvation of small ions (typically cations) dominates the hydration energy because of the 1/r2 dependence. For salts that contain large anions, EL does not change much as + changes. read more