Potassium is the element that has a first ionization energy of 418 kilojoules per mol. The first ionization energy refers to the amount of energy required to remove 1 electron from 1 mol of gaseous atoms.
The higher the ionization energy of an element, the higher attraction there is between its electrons and the nucleus of the atom. The ionization energy is based on how positively charged the nucleus is and how close electrons are to the nucleus. Other factors include the number of electrons between the nucleus and outer electrons, and whether an electron is on its own orbital or coupled with another electron.