Periodic table

• Elements are arranged in order of atomic number in the periodic table. So elements in the same periodic group have same amount of electrons on their outer shell, which gives them similar properties
• Early periodic table by John Newlands:

  • Ordered his table in order of atomic weight
  • Realised similar properties occurred every eighth element
  • This was called the 'law of octaves' but broke down after calcium

  
• Early periodic table by Dmitri Mendeleev:

  • Ordered his table in order pf atomic mass, but not always strictly (i.e. in some places he changed the order based on atomic weight)
  • Left gaps for elements he thought which hadn't been discovered yet
• Modern periodic table:

  • Elements are ordered by atomic number
  • Horizontal rows are called periods
  • Vertical rows are called groups
  • Elements with similar properties are in the same group
  • Elements predicted by Mendeleev were discovered and filled in the gaps