Ionization of air from a Van de Graff generator occurs because the electrons on the domb repel all the electrons in the nearby air. (This is just like if your holding the north poles of two magnets together they push away from eachother). With all the electrons pushed away it leaves the air positively charged or ionized. The electrons on the dome are then attracted to the air and leap off the dome to fill the holes that were left by the electrons that were just pushed away. The domb then starts recharging and the process starts over.

Electrons on the Van de Graff generators dome push against the electrons in the nearby air molecules.

The electrons in the nearby air molecules run away from the larger charge on the Van de Graff generators dome. Leaving the air positively charged or ionized.

When there is enough attraction in the surrounding air and or a path to ground is provided the electrons leap off the domb and through the air leaving the domb discharged, the nearby surrounding air neutralized, but the net not so distant air with an extra negative charge.

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