electrocuting blast

A beam of yellow electricity lances out toward a creature  that you can see within range. Make a ranged spell attack against the target. On a hit, the target takes 1d8 lightning damage. the target must make a constitution saving throw.On a failed save, the residual energy detonates in a static shockwave. the target is stunned until he finishes his turn. 

 At 5th level, the detonation deals an extra 1d8 lightning damage to the target on a hit, and the ranged attack damage increases to 2d8. Both damage rolls increase by 1d8 at 11th level (2d8 and 3d8) and again at 17th level (3d8 and 4d8).