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# 4.5 The Periodic Table

In the early 1800s, only about 30 elements were known. This was not enough to see any patterns in the chemical behavior of different elements. The invention of the battery gave a new way for compounds to be separated into their constituent elements. Several dozen new elements were soon recognized. Soon, the Swedish chemist Jacob Berzelius introduced the modern notation for compounds, based on the numbers of atoms (such as H2O). In 1869 the Russian chemist Mendeleyev arranged all the known elements into a table according to the weights of their atoms and by similar chemical properties. This is the basis of periodic table of the elements used in chemistry today. Mendeleyev’s organization of the elements by atomic weight firmly established the atomic theory of matter.

Jacob Berzelius. Click here for original source URL

Dmitri Mendeleev in 1897. Click here for original source URL

The periodic table provides a wonderful example of how science works. Notice that humans have been conducting innovative experiments in chemistry for thousands of years. Think of the invention of gunpowder or the many types of alcoholic beverage, for example. Our culture has been successfully building and manipulating chemical compounds without knowing anything about the "hidden structure" of atoms. For example, the Bronze Age was the first great age of technology when cultures moved beyond flints and fashioned a wide variety of tools and weapons from bronze. Bronze is a mixture or alloy of copper and tin, both of which are very malleable metals. How can the sum of two soft metals make a hard metal? Ancient cultures only knew that it worked. Now we know that soft metals have planes of atoms that easily slide over each other. It turns out that tin atoms provide the "grit" in the copper lattice that stop the planes from moving freely and give bronze its stiffness.

Modern periodic table of the chemical elements. Click here for original source URL.

Late in the 19th century, scientists began to see the patterns in the periodic table. Elements at the far right share the properties of being colorless and odorless gases that do not react with other elements: the noble gases. Elements at the far left are highly reactive, but they combine with elements in the seventh column to form stable compounds like common salt (NaCl). Scientists knew that the periodic table "worked," but they had no idea why it worked. Science often starts with this process of recognizing patterns in nature. Let’s use a playing card analogy. Someone who did not know the rules might watch a poker game for quite a while before figuring out the value of the different hands. We are also trying to solve a puzzle and figure out the "rules of nature."