the periodic law history of the periodic table. mendeleev’s periodic table classification of...
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The Periodic Law
History of the Periodic Table
Mendeleev’s Periodic Table• Classification of elements depended upon accurate
measurements of atomic mass.• Mendeleev’s first table was published in 1869.• Elements were arranged in order of increasing atomic
mass and similar chemical properties occurred at regular intervals.
• Approximately 60 elements were known at the time.• Mendeleev left empty spaces in his table and
predicted the properties of elements that had yet to be discovered.
Mendeleev’s Periodic Table
Unanswered Questions:
1. Why could most elements be arranged by increasing atomic mass but others could not?
2. What was the reason for chemical periodicity?
Moseley and the Periodic Law• (1911) – Mosely discovered that elements fit into
patterns when they were arranged by increasing nuclear charge (# of protons)
• The concept of atomic number became recognized as a means for identifying and classifying elements.
• This organization matched Mendeleev’s grouping by chemical properties
Periodic Law – physical and chemical properties of the elements are periodic functions of their atomic numbers.
The Modern Periodic Table
Noble Gases• Discovered between
1894-1898• Placed on the periodic
table between Group 17 and Group 1
• Unreactive elements
He
Ne
Ar
Kr
Xe
Rn
The Inner Transition MetalsLanthanides• Discovered in the early
1900’s• Similar chemical and
physical properties• Form 3+ ions
Actinides• Radioactive elements• Some are naturally
occurring; others have been created synthetically