Question

Which element has greatest hydration energy, sodium ion or potassium ion?

Answer 1

In row Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ hydration energy decrease.
This is due to increasing of ionic radius in this row.
For sodium ions hydration energy is 406 kJ/mol and for potassium ions it is 322 kJ/mol. So, answer is sodium ion.