In a simplification: thecalcium has released two electrons in becoming ionised to Ca2+. For the purposes of the calculation theoxygen picks up these two electrons one at a time. Putting one electron onto the oxygen (1st EA) actuallyreleases energy as oxygen has a high attraction for electrons, hence a negative delta H(arrow goes down). However gaining thesecond electron (2nd EA) requires an input of energy (positive deltaH, arrow goes up) as the electron is now being gained by an electron richoxygen ion.