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