Development of the Periodic Table

1. Development of the Periodic Table

2. Johann Doberiener (1829) • Arranged elements in groups of three (triads) • For example: Cl (mass 35) Br (mass 80) I (mass 127) Note: Average mass of the extreme elements =mass of the middle element

3. John Newland (1865) • Arranged elements based on the law of octaves. • Elements were arranged by increasing atomic mass • The chemical properties of the elements repeated every 8th element.

4. Lothar Meyer (1867) • Father of the Modern Periodic table. • Based on increasing atomic mass • Chemical properties of elements reoccur in a periodic fashion.

5. Dmitri Mendelyeev (1869)(Russian chemist) • Like Meyer prepared a table based on increasing atomic mass. • Consisted of 17 columns with 63 elements • Widely accepted • Constantly revised as new masses were discovered • Left vacant spots for new elements (Ga, Cd,Ge,Tc, Re, Po) • Certain discrepancies as new masses were found: Ar and K, Co and Ni, Te and I

6. Henry Mosley (1913) • English scientist used x-rays to determine atomic number (number of protons) • Discovered that the properties of elements were periodic functions of their atomic numbers, • Many of the discrepancies disappeared.

7. Present arrangement • Based on increasing atomic number • Periodic Law: properties of the elements depend on the structure of the atom and vary with the atomic number in a systematic way • Horizontal rows are called periods (#1-7) Indicates the number of principal energy levels. Note: Lanthanide series part of period 6—long period and Actinide series part of period 7—unfinished period Properties of elements change within a period • Vertical columns are called groups or families (#1-18) Indicates the number of valence electrons. Elements within a group have similar properties. Ex.Na and Li

8. Element Key