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The periodic table is a classification system for the
elements. Each element has unique properties and you
can predict an element's physical and chemical properties
based on its position in the periodic table.
 When
you look at the periodic table, the symbols and numbers
look like the image on the left. The symbols are abbreviations
of the origins of the word for each element. For example,
the origin of lead's symbol, Pb, is the Latin word 'plumbum'
meaning 'liquid silver'.
The number above the symbol is the atomic mass
(or atomic weight). This is the total number of protons
and neutrons in an atom.
The number below the symbol is the atomic number
and this reflects the number of protons in the nucleus
of each element's atom. Every element has a unique atomic
number. Lead has 82 protons therefore its atomic number
is 82. The elements are then arranged in order of increasing
atomic numbers - so hydrogen (H), which only has
one proton, is located first in the table, while sodium
(Na), which has 11 protons, and is found further down
the table.
We can use atomic number and atomic mass to find out
the number of protons, neutrons and electrons there
are in an atom. By subtracting the atomic number from
the atomic mass we can discover the number of neutrons
there are.
There are 18 main columns of elements
in the periodic table. These are called groups or families.
Elements within the same group or family have similar
- but not identical - properties. For example lithium,
sodium and potassium are members of family 1. They are
all highly reactive, and thus can easily combine with
other elements to form compounds. Each horizontal row
is called a period. Unlike elements in a family, elements
in a period are not alike in properties. There are 7
periods of elements. The 2 rows that have been separated
out of the main table are rare-earth elements, or lanthanons
(lanthanides) in the first period and actinons (actinides)
in the next period.
The journey towards the creation of
the periodic table can be traced as far back as the
Ancient Greeks of 400BC who used the words "element"
and "atom" to describe the differences between
things.
The periodic table we know today was
created by 4 separate European scientists during the
1860s. In 1863, 44 year-old French geologist A. E. Béguyer
de Chancourtois created a list of elements measured
by their atomic weight. Meanwhile, in England, chemist
John A. R. Newlands noticed that chemical groups repeated
every 8 elements. He named this the octave rule, comparing
it to musical notes.
 Between
1868 and 1869, working independently, Russian chemist
Dmitri I. Mendeleev and German chemist Lothar Meyer
arranged the elements into 7 columns relating to their
chemical properties. Their tables were similar, but
it was Mendeleev’s table that was adopted. It assembled
observations and discoveries of earlier chemists, showing
the symbols of the chemical elements in the order of
their atomic masses, but his table was a huge scientific
breakthrough because he left gaps and accurately predicted
undiscovered elements. This greatly accelerated the
discovery of new elements and our understanding of their
properties, and it enabled scientists to accurately
predict the elements – many elements have been
discovered or artificially made which have fitted into
the gaps in the table. Mendeleev predicted the properties
of scandium, gallium and germanium: within 20 years
scientists had discovered the existence of these elements
and his predictions were found to be accurate. Modern
versions of Mendeleev’s table have 109 elements,
but new
elements are being discovered all of the time.
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