 In this video we'll look at another property of atoms that exhibits a periodic trend, the ionization energy of an atom. Imagine the process of taking a neutral atom and removing an electron from it. The reason electrons stay in an atom is because they're negatively charged and experience an electrostatic attraction with the positively charged nucleus. That means that in order to remove an electron from an atom you have to put in sufficient energy to break that attraction. This process is called ionization and it's always endothermic. The first ionization for an atom is when you take the neutral atom and you remove one electron from it, which forms a plus one ion. As you remove successive electrons you also have the second ionization and the third ionization and so on, until you run out of electrons. The ionization energy is defined as the amount of energy needed to ionize a mole of atoms, so it's expressed in kilojoules per mole and different elements have different ionization energies. Potassium, this is potassium here burning in air, potassium's first ionization energy is 419 kilojoules per mole, which may seem like a lot, but the first ionization energy of gold is 890 kilojoules per mole, more than twice as much. In fact 419 kilojoules per mole is quite a low value for an ionization energy. That has a really important effect, because less energy is required to turn potassium into an ion it's much easier for potassium metal to undergo chemical reactions in which it becomes an ion and forms ionic compounds. As a result potassium is never found in nature as an element, only as part of compounds like potassium carbonate, potassium chloride and potassium hydroxide. If a neutral potassium atom were to be produced out there in the wild it would find something to react with like oxygen or water pretty quickly. Now consider gold, it's one of the few metals along with copper and lead that were discovered in their metallic state early in human history, and this is because they can be found in the earth in their native elemental state, not disguised in a compound but obviously shiny and metallic. And one of the reasons for this is that their first ionization energies are high, which makes it less likely that they will react with other chemicals around them.