First noticed similarities among certain elements in 1829

Similar elements seemed to appear in groups of three

Dobereiner’s Triads: ex. Cl, Br, I and Li, Na, K]

Arranged known elements in order of increasing atomic mass in 1864

Noticed similar chemical and physical properties

Newland’s Law of Octaves Problem: table had no gaps and

therefore left no room for new elements

Discovered periodic law in 1869 with Lothar Meyer but Mendeleev published his ideas first

Elements arranged in order of increasing atomic mass

Elements with similar properties recur at regular intervals

Gaps in the periodic table would later be filled by newly discovered elements (ie. Ga and Sc filled two of the gaps after they were discovered

Predicted properties of elements before they wer discovered

Caused periodic table to be modified by the discovery of Ar (noble gas) in 1894

Suggested in 1911 that the periodic table be arranged according to atomic number instead of atomic mass and thus eliminate several minor inconsistencies.

Example: according to atomic mass only, some elements appeared in the wrong column of the table, ie. Ar

In 1913, confirmed that the properties of elements depend on atomic number rather than atomic mass

RESULT: Our modern periodic table today

When elements are arranged in order of increasing atomic number, elements with similar chemical and physical properties occur at regular intervals.