Knowledge of the chemical elements - as opposed to the metaphysical
elements of ancient Greece, India, and China, dates back only
a few centuries. In the 18th century the French chemist 
Antoine
Lavoisier revived Robert
Boyle's use of the word "element" for substances that could
not be broken down into simpler substances. Over the next 75 years
many new elements were discovered, and some substances previously thought
to have been elements were actually shown to be chemical compounds.
It was also discovered some elements are similar to others. Sodium
and Potassium, isolated in the early 1800's by Humphrey
Davy, are both soft metals that react violently to water to
produce alkaline solutions.
One quality being catalogued at that time was atomic weight. Using various analytical skills, nineteenth century chemists gradually catalogued the atomic weights of various elements with more accuracy. the weight of hydrogen was taken as 1, with other elements related to it.
The Russian chemist Dmitry
Mendeleyev first proposed the modern periodic table of elements
in 1869. He arranged the elements in tabular form in order of increasing
weight. Where there were large gaps between two elements he assumed
a new undiscovered one. As the elements were also arranged in families
(columns) this enabled him to predict the properties of the undiscovered
elements with great accuracy (like Sodium and Potassium referred to earlier)
This major theoretical breakthrough provided the necessary classification
system by which the properties of all the atomic elements could be mapped
out.
Element, chemical - excellent basic introduction at Science Clarified to the chemical elements; includes an explanation of the periodic table
Periodic table - on wikipedia. Includes links to each element
The
Periodic Table of Chemical Elements - a basic html table - lists
all the elements
The nuclei of hydrogen and most helium atoms were formed about one microsecond and three minutes respectively after the original Big Bang. Heavier elements were made later, by nuclear reactions in the core of stars - the heaviest reactions being formed inside exploding stars.
It is estimated that the rate of element formation peaked sharply at around 11,000 million years ago. Galaxies around 6000 million light years distant (redshift z=0.5) show more blue, element making stars than nearer (closer to us in time) galaxies do.
The average age of those elements heavier than hydrogen, found in meteorites
and on earth, is, according to 
radiometric
dating, around 10,000 million years, although there were significant
late injections from other exploding supernovae at 4650 ago and 4,550 million
years ago, shortly before the formation of the Solar
System.
The average age of elements in the spiral arms of the galaxy, where element making has continued during the past 4,550 million years, is around 9,000 million years. In globular clusters and in the center of the galaxy, elements may be somewhat older.
Nigel Calder, Timescale - an Atlas of the Fourth Dimension , pp.226-7
(Hogarth Press, London)
the mandala of the elements
from Edward Haskel, ed. Unified
Science
CHAPTER
7: ENERGY- Chemistry, Crises and the Chakras- by Arnold Keyserling
and R.C.L. - gives a metaphysical approach to the elements and periodic
table
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