2. 14 elements were known

3. Johann Dobereiner (German Chemist) noticed that Br, Cl, and I had similar properties and that Br’s atomic weight was between that of Cl & I. He found three other groups with similarities. He called these groups triads.

4.  Cl, Br, I  Ca, Ba, Sr  S, Se, Te  Li, Na, K

5. John Newlands (English Chemist) arranged the now known 62 elements from lightest to heaviest. He also noticed that every 8 th element had similar chemical and physical properties. Their properties were repeating. This became known as Newland’s Law of Octaves.

6. Dmitri Mendeleev (Russian Chemist) organized the elements by atomic weight also but made it into table form to help his students. Elements with similar properties were put into the same column.

7.  He left blanks where elements seemed to be missing.  There were places where heavier elements were put before lighter elements because of properties: Te – I Co – NiAr – K (Mendeleev was ridiculed for this because it disobeyed the current Periodic Law, “Properties of elements are periodic functions of their atomic weight.”)

8. Henry Moseley (English Chemist) arranged elements by atomic number. This gave rise to a new Periodic Law, “Properties of elements are periodic functions of their atomic number.”