Periodic Table

• Early 1800s:

  • Elements were categorised by:

  1. Physical and chemical properties 2. Atomic weight

  • Elements were arranged in order of atomic weight
  • Early periodic tables were not complete and some elements were placed in wrong group - elements were placed in order of atomic weight and properties were not taken into account
• Dmitri Mendeleev:

  • Mendeleev created his table in 1869
  • Mendeleev put elements mainly in order of atomic weight but switched order if properties meant it should be changed
  • Gaps were left in table to make sure elements with similar properties stayed in same groups
  • Gaps indicated existence of undiscovered elements and allowed Mendeleev to predict their properties
• Modern Periodic Table:

  • Elements with similar properties form columns
  • Vertical columns form groups
  • Elements are laid out in order of increasing atomic number
  • Rows = periods - each new period represent full shell of electrons