The question we want to address in the rest of this manuscript is: "What is the origin of the peculiar electronic structure in HPEs?" But, before we do so, it is useful to review models that have been put forward to understand the electronic structure and bonding in wellstudied 1 atm systems conceptually related to electrides. The first of these to be explored experimentally dates back to the 18 th century, when Chemistry was evolving into a proper science. One of the key contributions of Sir Humphry Davy, a leading figure in Chemistry's development, was the isolation and discovery of several elements including potassium and sodium. To prove that potassium was an element, Davy reacted it with gaseous ammonia.