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Dobereiner ­ triads (German)
1800's ­ relative atomic mass only ­ known
arranged in order of relative atomic mass
1828 ­ he put list into groups of 3 (triads) ,
based on chemical properties
E.g. Cl, Br, I
E.g. Li, Na, K
The middle element = relative atomic mass
average of the two Element Relative atomic
Lithium 7
(7+39) / 2 =23 Sodium 23
Potassium 39…read more

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Newlands ­ Law of Octaves (sets of 8)
1864 ­ every 8 element had similar properties: listed
in rows of 7
But, pattern broke on 3rd row ­ transition metals e.g.
Ti, Fe
He left no gaps = work ignored
1965 - Presented ideas to the Chemical Society, but
Groups contained elements that didn't have similar
properties e.g. C and Ti
He mixed metals + non-metals e.g. O and Fe
Didn't leave gaps for elements not yet discovered…read more

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Dmitri Mendeleev ­ gaps + new elements
1869 - arranged 50 known elements into his Table of
Elements + gaps
Elements in order of atomic mass (like Newlands) +
gaps, so that elements are in similar properties in the
same vertical groups ­ left very big gaps in 1st 2 rows
before transition metals in 3rd row.
Gaps predicted properties of undiscovered elements ­
when found and fitted = good news
Made even more sense when later discoveries made
on atomic structure:
Each element has an atomic no. one more than previous
Pattern in periodic table ­ 2 elements in 1st row, 8 in 2nd, 8
in 3rd matches the electron arrangement in an atom…read more


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