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History.
The Periodic
Table came about through attempts by people
to group elements according to their chemical
properties.
John
Newlands (1863) noticed that by
arranging the elements
in order of increasing atomic mass
(mass number)
every eighth element seemed to have
similar properties.
He proposed a similarity with
music,
where the eighth note is an
octave above the first.
This became known as Newlands
Octaves
but did not work for the fourth
period with the
transition
metals.
It works for the lighter elements
(atomic number less than
19)
because eight electrons completes the outer shell.
Dmitri
Mendeleev is credited as being the Father of the modern
periodic table.
In 1869 he arranged the 50 or so known
elements in order of atomic
mass,
putting elements with similar properties in the same vertical group,
and leaving gaps for unknown elements, yet
to be discovered.
When the elements were later
discovered,
they were found to have the properties
predicted by Mendeleev's
table.
Knowing nothing of protons, nuclei or atomic number,
Dmitri Mendeleev's periodic table was
broadly correct (genius!).
The modern periodic table is very useful for giving
a summary of the atomic
structure of all the elements.
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