History of the Periodic Table - two main figures in Periodic Table


​Mendeleev – arranged his table by  Atomic Mass

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​Moseley: Modern Table – arranged his table by Atomic Number
Atomic number represents the number of protons in an atom.

Arrangement of the Periodic Table

Columns (across the top):     
What are the columns called?     groups
What does each element in a column have in common?  number of valence electrons (last shell);  similar chemical properties 

​Rows (down the side):     
What are the rows called?  periods
What does each element in a row have in common?  shells (principle energy levels)


Group Names
1: Alkali Metals: very reactive (1 valence electron)
2:    Alkaline Earth Metals: (2 valence electrons)
3-11:    Transition Metals: ions in colored solutions
17:    Halogens: (7 valence electrons)
18:Noble Gases:Complete outer shell-non reactive (Monomers)