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Periodic Table History
From Dobereiner to the Present
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J.W. Dobereiner Early 1800s, German
Elements classified into groups of 3: triads Elements in triad have similar properties Properties of middle element are average of the first and third elements in triad
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J.A.R. Newlands 1800s, British Elements arranged in order of increasing atomic mass – pattern repeats every 8 elements Law of octaves
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Dmitri Mendeleev Russian, 1800s; Lothar Meyer developed concurrently
Organized elements by increasing atomic mass Elements with similar properties grouped together Had some discrepancies, but predicted properties of undiscovered elements
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Henry Moseley Early 20th century, British
Organized elements by atomic number This solved discrepancies in Mendeleev’s periodic table
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Groups Vertical columns in table
Elements in a group have similar chemical properties
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Periods Horizontal rows in table are called periods
Represent energy level or principal quantum number, n
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Organization of Elements in Modern Periodic Table
Metallic Character Metals Nonmetals Metalloids
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Organization (Continued)
Blocks Main Group Elements Transition Metals Inner Transition Metals
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Numbering IUPAC and American Numbering Systems
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