Periodic Table
The periodic table you see is the work of almost 200 years. The modern periodic table
has roots in the first periodic table designed by the Russian Chemist, Dmitri Mendeleev
(1834-1907). By the way, his picture on the left is taken from wikipedi
(http://en.wikipedia.org/wiki/Dmitri_Mendeleev). In his time, only 65 elements were
known and he was the first one to systemtically arrange these elements in the form of a
table. There are two very important aspect of his table:
• When the elements were arranged in order of increasing mass, certain properties
recur periodically. This is known as the periodic law or periodicity.
• He left some empty spaces in the table for the elements yet to be disciverd. In
fact, these elements were discovered later and filled the empty spaces.
First of all, the periodic table is a two-dimensional chart. It is a systematic arrangement of
elements in order of increasing atomic number as well as atomic mass (do not get
confused this terminology with mass number). What does it mean? It means that when
we move on from one element to next element, the atomic number as well as atomic mass
increases. However, there are few exceptions here and there in the periodic table in terms
of atomic mass, it decreases instead of increases, for example 27Co (mass= 58.93) and
28Ni (mass = 58.69). There is a great periodic table that is available from American
Chemical Society. Click on the link to view it. And also, click on any box, to see the data
on that element. When elements are arranged in this fashion, the elements falling in the
same column exhibit similar chemical properties. This is the periodicity.
The periodic table is dissected in various ways and these dissected pieces have the
following special name
• Periods: The horizontal rows (run from top to bottom).
• Groups or families: Vertical columns (run from left to right).
Groups
Periods
Elements are classified as either A group elements or B group elements:
• Representative elements: A group elements (these are on either side of the
table).
• Transition elements: B group elements (these are in the middle of the table).
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