Chapter 7 - The Periodic Table of the Elements.
 History of the Periodic Table of the ElementsAs you remember from the last chapter, Dalton developed the idea of atoms in 1803 because so many elements had been found that could not be broken down.By the 1860’s over 60 elements had been discovered and their properties described.   The problem was they were not organized.Dmitri Mendeleev placed the names and properties of the elements on small cards and tried to find a way to organize them. (sort of ‘elementary solitaire’)  When he placed the elements in order by their atomic mass he found that there was a repeating pattern of properties.  By taking the beginning of each pattern and starting a new row he found that they could be placed in an order that made sense.  He also discovered that he had to put a space in one row to make it fit!  He decided that the element that belonged in that space had not yet been discovered and described some of the properties of the element by its place in the periodic table.  (He did have to take two elements that had nearly the same atomic mass and switch their places to make the table work, but we will see why in the next paragraph)In 1914 Henry Moseley discovered the existence of the proton and the atomic number was born.  Before this the nucleus was thought to be made of a solid, positive substance.  With the discovery of the proton the elements could be put in order by their atomic number instead of their mass.  Now the elements fit into the table perfectly.This led to the development of the periodic law, which says that the properties of an atom come from the number of protons in the nucleus.  (or as we said, “the number of protons determines the type of element it is”)
 Organization of the Periodic TableYou first notice that the periodic table is made up of smaller squares, one square for each element.  In each of these squares you will information about the element.  Some tables will contain more information than others, but the will all contain the following.Chemical Symbol – one or two letters that tell you want the element is.Atomic Number – the number of protons in the nucleus of the elementAtomic Mass-the average mass of that type of element.There are three main classes of elementsMetals – these conduct heat and electricity and tend to be malleable and ductile.  They are found on the left side of the periodic table.Non-Metals – these do not conduct head or electricity well, they tend to be brittle.  They are found on the right side of the table.Metalloids – Located on a stair-step line between metals and non-metals. They have some metal and some non-metal qualities.Each row is called a period (thus the name periodic table) a period is a repeating pattern.Each column is called a group, all the elements in a group have some similar properties.