5-1: History of the Periodic Table

1. 5-1: History of the Periodic Table Objectives: Explain the roles of Mendeleev and Moseley in the development of the periodic table Describe the modern periodic table Explain how the periodic law can be used to predict the physical and chemical properties of elements Describe how the elements belonging to a group of the periodic table are interrelated in terms of atomic number

2. History of the Periodic Table

3. Mendeleev and Chemical Periodicity • Russian chemist Dmitri Mendeleev • Hoped to organize elements according to their properties • Placed name of element on card with physical and chemical properties • Arranged them according to various properties and looked for trends or patterns • Noticed when elements were arranged in order by increasing atomic mass, certain similarities in chemical properties appeared at regular intervals

4. Created a table where elements with similar properties were grouped together (periodic table of elements!) • Procedure left several empty spaces in his periodic table • Predicted the existence and properties of elements that would fill three of the spaces (1871) • All three elements discovered by 1886 • Scandium, Sc • Gallium, Ga • Geranium, Ge

5. Mendeleev’s fame came from his ability to predict the properties of the missing elements • Persuaded chemists to accept his periodic table • Earned him credit as the discoverer of the periodic law

6. The Genius of Mendeleev and the Periodic Table

7. Moseley and the Periodic Law • English scientist Henry Moseley (1911) examined the spectra of 38 different metals and recognized a previously unrecognized pattern • Elements fit into patterns better when arranged in increasing order according to nuclear charge (the number of protons) • Led to modern definition of atomic number and recognition that atomic number is the basis for organization of the periodic table

8. Discovery consistent with Mendeleev’s ordering of periodic table by properties rather than strictly by atomic mass • Mendeleev principle of chemical periodicity is correctly stated in what is known as periodic law • Periodic law: the physical and chemical properties of the elements are periodic functions of their atomic numbers

9. The Modern Periodic Table • Periodic table: an arrangement of the elements in order of their atomic numbers so that elements with similar properties fall in the same column, or group • Three sets of elements added to the periodic table after Mendeleev’s time • The noble gases • The lanthanides • The actinides

10. The Noble Gases • Most significant addition to the periodic table • Came with the discovery of the noble gases • All rather unreactive elements

11. The Lanthanides • Lanthanides: the 14 elements with atomic numbers from 58 (cerium, Ce) to 71 (lutetium, Lu) • Process of separating and identifying them was tedious because these elements are so similar in chemical and physical properties

12. The Actinides • Actinides: the 14 elements with atomic numbers from 90 (thorium, Th) to 103 (lawrencium, Lr)

13. Periodicity • Can be observed in any group of elements in the periodic table • Reason for periodicity explained by the arrangement of the electrons around a nucleus Periodicity: the quality or character of being periodic; the tendency to recur at intervals What are some other examples of things that you can think of that occurs periodically?