C1 - Development of the periodic table

• In the modern periodic table...

  • Elements in the same group have the same number of electrons in their outer energy level
  • Elements are arranged in the order of atomic number (the number of protons)
• How did John Newlands order the elements?

  • Arranged the elements by increasing atomic weight
• What was the problem with Newlands' 'Law of Octaves'?

  By using the exact order of atomic weight, sometimes elements were grouped together when they had different properties
• How did Mendeleev initially order the elements?

  Arranged by increasing atomic weight
• What did Mendeleev do afterwards?

  1. He switched the order of specific elements - so they fitted the patterns of other elements in the same group
  2. He left gaps in his periodic table for undiscovered elements
• Why did other scientists accept Mendeleev's table?

  The missing elements were discovered, and their properties matched Mendeleev's predictions
• Why did Mendeleev switch the order of elements?

  • Protons had not been discovered yet
  • So, he ordered the elements by atomic weight
  • However, elements can appear in the wrong order due to the presence of isotopes
  • So, Mendeleev switched the order of elements when he needed to
• Give the reasons why Mendeleev reverse the order of some pairs of elements

  So their properties matched the rest of the group
• Why did Mendeleev not order the elements in order of Atomic number?

  • Atomic number is the number of protons
  • Protons were not discovered until later