C1 - Development of the Periodic Table

Early 1800s:

• Elements were arranged in order of relative atomic mass.
• Scientists had no idea of atomic structure or subatomic particles so there was no such thing as atomic number to them.
• Early periodic tables were incomplete and some elements were placed incorrectly because elements were placed in order of relative atomic mass and didn't take into account their properties.

Dmitri Mendeleev:

• Left gaps.
• Mainly ordered according to atomic masses, but switched them if the properties meant it should be changed.
• Gaps were left to ensure that elements with the same properties were together .
• He made predictions about the properties of undiscovered elements and when they were found to fit the pattern, it helped confirm his ideas.