History of Periodic Table
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In 1869 the chemist Dimitri Mendeleev began making the periodic table
Mendeleev also arranged the elements known at the time in order of relative atomic mass, but he did some other things that made his table much more successful.
He put the elements into a grid with increasing atomic weights and noticed the elements in each column had similar chemical properties.
Elements with similar properties appeared under each other. Gaps were left for yet to be discovered elements.
The periodic table was arranged by atomic mass, and this nearly always gives the same order as the atomic number. However, there were some exceptions (like iodine and tellurium), which didn't work. Mendeleev had seen that they needed to be swapped around, but it was Moseley that finally determined why.
We group the similar elements into columns instead of rows. ... All the elements in the seventh column have 7 electrons in their outermost shell. And so, scientists finally found the reason for the periodicity of the elements: Valence electrons. And so the table became known as the Periodic Table.
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