We will finish our discussion of periodic trends by learning about electron affinity, which describes how much an element wants to get an extra electron. Some elements, such as the halogens, will do anything for another electron and have the highest electron affinities. Other elements like noble gases wouldn't take an electron if you handed it to them on a silver platter. As a reminder, electron affinity is the opposite of ionization energy. Electron affinity is usually exothermic, especially when an element can fill a sub-shop. Elements which want to gain electrons therefore have negative electron affinities. We will see that the trend for electron affinity is not as clear cut as the periodic trends for size or ionization energy. In general, electron affinity decreases down a column, though there are many exceptions. Electron affinity also generally increases to the right across a row, but there are many exceptions. This means that the elements with the greatest electron affinity live at the upper right of the table except not the noble gases. The elements with the lowest electron affinity live in the lower left of the table, but also include the noble gases. This visualization of electron affinity shows that the general trend has many exceptions. However, it does a good job showing the difference between metals and non-metals. Electrons have low electron affinities and would much rather get rid of their electrons to form cations. Non-metals have higher electron affinities and like to gain electrons to form anions. Look closely and you can see that elements with full or half full sub-shows have low to know electron affinity. This is apparent at the end of the s block, at the end of the d block, and halfway through both the d block and the p block. But it is most apparent for the noble gases, which have absolutely no electron affinity due to their completely full valence shown. This slide summarizes the three periodic trends explored in the last few sections. The periodic trends explain the reactivity of elements on the table, especially the difference between metals and non-metals. Reactive metals have low ionization energies and small electron affinities. These elements are not holding onto their electrons very tightly, giving them the properties of conductivity and easy oxidation. Reactive non-metals have high ionization energies and large negative electron affinities. These elements become more stable after they found a way to steal or share more electrons. These elements frequently form molecules and act as good insulators to electricity. Noble gases have a completely full outer electron shell, giving these elements very high ionization energies and positive electron affinities. The noble gases neither lose nor gain electrons, making them the least interesting of all the elements.