Lesson Plan - Get It!
Why do you think the periodic table is organized the way it is?
The periodic table helps us organize all 118 elements we know of (as of 2017).
Before continuing, if you missed or want a refresher on the previous Related Lessons in this series, find them in the right-hand sidebar.
It uses a straightforward organization method, placing the elements in order based on the number of protons. Since protons are used to identify each element, this reduces confusion for scientists using the periodic table around the world. The periodic table wasn't always so organized, and we haven't always had so many elements. As you can imagine, our understanding of elements has changed as the tools we use to study science improve.
From very early times, humans knew of elements like gold, silver, lead, and copper. By the 1860s, scientists had discovered around 60 elements, but they did not have a standardized way to organize this information. The first proposed organization grouped elements with similar observable characteristics in a spiral formation on a sheet of paper, and was created by Alexandre-Emile Beguyer de Chancourtois, a French scientist. It was called the "telluric screw" because of the placement of the element tellurium. While the model took atomic weights into account, it did not provide an accurate representation of element placement.
Image by Alexandre-Emile Béguyer de Chancourtois, via Wikimedia Commons, is in the public domain.
Two years following the development of the spiral model, an Englishman, John Newlands, proposed a new order. He noticed a trend when elements were written in order by atomic weight: a pattern emerged. This led to the "law of octaves," that states that elements organized by weight demonstrate similar characteristics after each interval of eight elements. This was one of the first periodic trends noted on the periodic table.
- Do you think similar observations are reflected on the periodic table today?
The image below shows these relationships:
Image by Sponk, via Wikimedia Commons, is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
In 1869, Dmitri Mendeleev, a Russian scientist, proposed a table that organized the elements based on atomic mass. In this periodic table, elements with similar characteristics were arranged under one another in groups. Mendeleev left gaps for elements that had not yet been discovered, based on patterns he observed in the elements.
Image by Dmitrij Mendelejev, via Wikimedia Commons, is in the public domain.
Mendeleev is the father of the modern periodic table. Elements continue to be added to his basic model today.
In 1914, Henry Moseley changed how the elements were organized on the periodic table. He was able to determine the atomic number for each element, and concluded that organizing elements by atomic number instead of atomic mass was more efficient. It provided a clearer representation of chemical relationships among elements. This is the organization we use today.
The periodic table has experienced many changes over the years as our understanding of science and chemistry deepened. The first organization of elements was simple, and contained only a few elements. As we learned more about elements and the atomic structure, we were able to improve the method of organization. Today, we use the number of protons, which nicely places each element in order.
- Why do you think it is important for the elements to be neatly ordered?
- How might this help scientists using the periodic table?
Respond to these two questions on a sheet of paper before moving to the Got It? section, where you will learn more about how elements were discovered.