Electrochemical series is a series of elements arranged in increasing order of their reduction potential and  $E_{H^{+}/H_2}^0=0$. The metals above $H_2$ have  $–ve$ reduction potential, they are more reactive than hydrogen whereas metals below hydrogen are less reactive than $H_2$. Reduction potential of metal depends upon  (i) sublimation energy, 
(ii) ionization energy and (iii) hydration energy of ions.