Headings Search Questions 
History.
The Periodic
Table came about through attempts by people
to group elements according to their chemical
properties.
Dobereiner (1829)
found that with some groups of three
elements
with similar properties, for example lithium,
sodium and potassium,
the atomic mass
(mass number) for the second
element
was the average of the first
and third element.
So, lithium has a mass
number of 7 and potassium
has a mass number of 39.
The average is (7 + 39)
÷ 2 = 23.
Sodium has a
mass number of 23.
The same was found for some other groups of three
elements
for example carbon
(12), nitrogen
(14) and oxygen
(16).
These groups became known as Dobereiner's
Triads.
John
Newlands (1863) noticed that by
arranging the elements in order of
increasing atomic mass
every eighth element seemed to have
similar properties.
He proposed a similarity with
music,
where the eighth note is an
octave
above the first note.
This idea became known as Newlands
Octaves.
It 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.
gcsescience.com Contents The Periodic Table Index Quizzes gcsescience.com
Copyright © 2011 Dr. Colin France. All Rights Reserved.