IT IS SIMPLE
IONIZATION ENTHALPY MEANS WHAT : ENTHALPY TO LOSE e TO GET +1,+2,+3 OXIDATION STATE i.e. SHOWING METALLIC CHARACTER .
SAME WAY WITH I GAIN ENTHALPY............
ELEMENT HAVING MOST METALLIC CHARACTER WILL HAVE THE MAX TENDENCY TO LOSE e THIS MEANS ITS IONIZATION ENTHALPY WILL BE LESSER .
I HOPE THIS WILL SOLVE YOUR QUERY.. (IF FURTHER DOUBTS THEN JUST NUDGE ME)
RATE ME PLZZZZZZZZZ. IF U FIND IT USEFUL.