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The Periodic Table

The History of the Periodic Table.

The periodic table came about through attempts by people
to group elements according to their chemical properties.
It is called periodic because similar chemical properties
of the elements were found to occur at regular intervals.

What are Dobereiner's Triads?

Dobereiner (1829) found that some groups of three elements
that have 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.

For example, lithium has a mass number of 7 and potassium
has a mass number of 39. The average is (7 + 39) ÷ 2 = 23.
The mass number of sodium is 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.

What are Newlands' Octaves?

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.

What did Dmitri Mendeleev do?

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 unknown 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 and the electron structure of all the elements.

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