Learning objectives Know the history of the periodic table

1. Learning objectives • Know the history of the periodic table • Understand how chemical and physical properties of the elements led to the development of the periodic table • Development of the Periodic Table

2. Some elements have been known since ancient times • Copper (Cu) • Gold (Au) • Lead (Pb) • Mercury (Hg) • Silver (Ag) • Tin (Sn) • Early Elements Mercury

3. Limited ability to perform scientific research caused no new elements to be discovered for thousands of years • Scientific experimentation increased during the Enlightenment, additional elements were discovered • First chemistry textbook was published in the 1780s by Lavoisier and listed the 23 elements known at that time • Next Wave of Discovery http://www.schuster-ingolstadt.de/Chemie.htm [public domain]

4. By 1869, there were 63 known elements • Mendeleev organized the elements by their atomic masses and chemical reactivities • Created the first widely accepted periodic table • Mendeleev’s Periodic Table • Portion of Mendeleev’s Original Table By Den fjättrade ankan at sv.wikipedia [Public domain]

5. Mendeleev found patterns by arranging cards with the names, masses, and properties of elements on them • Grouped elements that had similar “combining powers” • Alkali metals had a combining power of one • Group I • Alkaline earth metals had combining power of two • Group II • Copper and mercury were difficult to classify since they combined in multiple ways • Now classified as transition metals • Hydrogen was difficult to classify since it had properties of the alkali metals and the halogens • Still difficult to place on modern periodic tables • Patterns in Mendeleev’s Table

6. To arrange elements, Mendeleev relied more on similarities in chemical and physical properties than accepted masses • Exceptions to arrangement by mass • Argon and potassium • Cobalt and nickel • Tellurium and iodine • Left spaces for undiscovered elements • Correctly predicted their chemical reactivities and masses • Gallium, scandium, and germanium • Mendeleev’s Insights

7. Additional work led to discovery of various inert gases • Helium • Neon • Argon • Krypton • Xenon • Placed into their own group, since they did not react with other elements Portion of Mendeleev’s Table, 1905 • Discovery of Noble Gases By Д.И.Менделеев (http://ether-wind.narod.ru/Mendeleev_1905/) [Public domain]

8. In 1914, Henry Moseley’s work with x-rays showed each element had a unique positive charge in the nucleus • Atomic number – number of protons in an atom • When Mendeleev’s table was organized by atomic number, problems caused by organization by mass disappeared • Moseley’s atomic numbers confirmed the existence of predicted elements technetium and promethium • Arranging by atomic number also eliminated problems associated with isotopes of the same element having different atomic masses • Arranging Elements by Atomic Number

9. Periodic law – when elements are arranged according to atomic number, properties of the elements repeat at regular intervals • Modern periodic table is arranged by atomic number • Periodic Law

10. Bohr’s and Lewis’ work on the electronic structure of the atom confirmed organization of the periodic table • Repeating chemical properties are caused by the number of outer shell electrons of each element • Contributions by Seaborg led to the separation of the lanthanides and actinides as their own group • Modern Periodic Table