Development of the periodic table

• How were elements arranged in the early 1800s?


  Chemical elements were arranged by atomic mass in the early 1800s.
• Atomic weight was used to categorise elements in the early 1800s because there was no knowledge of atomic structure.
• What is ‘atomic weight’ better known as now?


  Relative atomic mass
• Early periodic tables were incomplete and some elements were placed in the wrong group. This is because they were placed in order of atomic weight with no properties taken into account.
• Who invented the modern periodic table?

  Dmitri Mendeleev
• How did Mendeleev form the start of the modern periodic table?


  By arranging elements in order of increasing atomic mass and leaving gaps for undiscovered elements, but also taking properties into account.
• Gaps were left to ensure elements with similar properties that were yet to be discovered could still stay in the right group.
• Why was Mendeleev correct to place elements in the table based on properties as well as atomic weight?


  Because of isotopes which have the same properties as the original.