The Periodic Law

1. The Periodic Law History of the Periodic Table

2. 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.

5. Mendeleev’s Periodic Table Unanswered Questions: • Why could most elements be arranged by increasing atomic mass but others could not? • What was the reason for chemical periodicity?

6. 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.

8. The Modern Periodic Table He Noble Gases • Discovered between 1894-1898 • Placed on the periodic table between Group 17 and Group 1 • Unreactive elements Ne Ar Kr Xe Rn

9. The Inner Transition Metals • Lanthanides • 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