|History of the Periodic Table of the
As 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
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 Table
You 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
There are three main classes of elements
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.