World Builders™                                                                       Session One  --  Astronomy
   
The Periodic Table

      It was difficult to find out which substances were elements. Water is plentiful, but it is made up of two gases, oxygen and hydrogen. Who would have guessed that? The air is made up of a mixture of different gases, but that is not obvious. As the science of chemistry developed, scientists discovered a number of ways to take compounds apart. However, it was difficult to see how their knowledge could be systematized.

      In the 19th century John Dalton, a British schoolmaster, showed that the elements always occurred in consistent proportions in specific compounds. This made it possible to make sense of the idea that matter was made of atoms. Elements can be parts of compounds that are very different from each other (oxygen is part of both water and rust) but they behave in consistent ways in their interactions with each other.

  The next advance came from weighing equal volumes of gases and comparing those weights against oxygen, which was given a weight of 16. The weights came out pretty close to whole numbers, which certainly provoked thought. The Russian chemist Dmitry Mendeleyev proposed arranging the elements in the order which we now call the Periodic Table. It arranges the atoms of the elements by weight. The vertical rows group elements which have similar properties.

      Although over a hundred elements have been identified, the largest atoms are unstable (radioactive) and tend to break apart. As it turns out, atoms are thought to be made of smaller parts: protons, neutrons, and electrons. Each element has its own number of each of these little building blocks. Because of this regularity, it is reasonable to believe that matter all over the universe will be made of the same elements that we have here. These elements will form familiar compounds -- everywhere.  

Click on a Symbol to Learn About the Elements. Can you find some that you are familiar with?