Periodic Table was first discovered in
1869 by Dmitry Mendeleyev. The elements are
arranged in an increasing order of their atomic numbers as one goes
from left to right accross the table. The horizontal rows are called
periods and the columns are called groups.
gas is found at the right hand side of each period. There is a
progression from metals to non-metals across each period. Elements
found in groups (e.g. alkali, halogens) have a similar electronic
configuration. The number of electrons in outer shell is the same as
the number of the group (e.g. lithium 2·1).
The block of elements
between groups II and III are called transition metals. These
are similar in many ways; they produce colored compounds, have
variable valency and are often used as catalysts. Elements 58
to 71 are known as lanthanide or rare earth elements. These
elements are found on earth in only very small amounts.
90 to 103 are known as the actinide elements. They include most of
the will known elements which are found in nuclear reactions.
The elements with larger atomic numbers than 92 do not occur
naturally. They have all been produced artificially by bombarding
other elements with particles.