 There are many, many patterns in the periodic table, and many mysteries in it still too. It's like a condensed reference book for chemistry, but with room for more chapters to be added in the future. For a chemist, the more chemistry you know, the better you are able to read it. And the better you are able to read it, the more chemistry you'll know. In this video we're going to look at one particular pattern, the radii of atoms. That is how big they are. The pattern of the atomic radii is known as a periodic trend because we're using the periodic table, and because in the pattern you can detect trends in the measurements when you look at different parts of the table. Just as a quick reminder, horizontal rows in the periodic table are known as the periods. Atoms in the same period have the same outermost electron shell, the same valence shell, but they'll have different numbers of valence electrons in it. Vertical columns on the table are known as groups. Each atom in a group will have a different electron level as its valence shell, but they'll have the same number of valence electrons in that shell. These relationships between the internal structures of different atoms are important because the structure of an atom determines its characteristics, like its size and its charge, when it forms an ion. And these characteristics determine the properties and behavior of materials on a larger scale.