Development of the Periodic Table

Chemistry 5(A)

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

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

Early Elements

Mercury

• 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]

• 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]

• 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

• 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

• 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]

• 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

• 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

• 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

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