I've learnt in class that charges store up on capacitor plates (the electrons). But why don't the electrons just jump across capacitor, what stops them? At first I thought perhaps it might just be the air molecules, as they are insulating and have no charge carriers, the electrons wouldn't have enough energy to barge their way through. 

How about if I had a capacitor in a vacuum though? Would the electrons just be able to leap across the capacitor because there is nothing there to stop them? If this were the case however, the capacitor could theoretically be any separation and the electrons would simply float across, which seems absurd, so there must be something stopping the electrons from leaving?