Periods
This doesn't work for six elements, however. These being:
iron, manganese, nickel, cobalt, zinc and copperIn 1817, Johann Wolfgang Dobereiner thought that he had made a great
observation about elements.
He thought that there was a relationship between the atomic weights
of elements and their properties.
His theory was that, in groups of three (triads), the middle element had
the mean atomic mass of the two elements to either side of it.
This works with many elements, like Lithium, Sodium and Potassium.
(Li + K) / 2 = Na (7 + 39) / 2 = 23Dobereiner's TraidsIn 1864, John Newlands ordered the periodic table by relative atomic
mass.
After doing so, he regarded that each element was similar to the element
8 places further on.
This is true for some many elements, such as Lithium and Sodium
The name comes from musical instruments like the piano. There are
8 different notes in a piano, and moving up 8 notes is called an octaveNewlands' OctavesThere are many exceptions to this rule, however, including iron being
put in the same group as sulfur and oxygen - explaining why it was
disregarded by other scientistsIn 1869, Dmitri Mendeleev put forward another periodic table.
This table was published only 5 years after Newlands' and also organised
the elements in order of relative atomic mass.
Mendeleev took Newlands' ideas and furthered them, making the
connection between both chemical and physical properties and their
atomic masses.
This let him see that these properties were related in a 'periodic' way and
ordered the table using periods - aka vertical columnsMendeleev's Periodic TableThis was very accurate and precise and, in places where there were
exceptions, allowed for him to predict where new elements would be
placed and their relative atomic mass, to a great degree of accuracy
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