Periodic Table

• Modern arrangement of Periodic Table

  • Increasing atomic (proton) number
• Groups

  • Elements with similar properties placed in columns, known as groups
  • Have the same number of outer electrons
• Periods 


  • number of shells an element has
  • horizontal
• Why is it known as the 'periodic' table?

  • Similar properties occur at regular intervals
• Why is lithium in the same group as sodium?

  1. Same number of electrons in the outer shell
  2. similar (chemical) properties
• Early periodic table : Before protons, neutrons, electrons 


  • Ordered in increasing atomic weight
• Problems with early periodic table

  • Incomplete
  • Elements placed in inappropiate groups due to atomic weight
• Mendeleev periodic table

  • Left gaps for undiscovered elements
  • Changed the order based on atomic weights
• What led to the acceptance of Mendeleev's table?

  • Predicted properties of missing elements
  • Elements were discovered which fitted the gaps
  • Elements with properties predicted by Mendeleev were discovered
• John Newlands Table VS Mendeleev's Table

  Mendeleev :

  • left gaps for undiscovered elements
  • Changed the order of some elements

  John Newlands :

  • Many dissimilar elements in the same column
  • Did not leave gaps for undiscovered elements

  BOTH :

  • both missing the noble gases
  • both arranged elements in increasing atomic weight
• Why were the noble gases missing from Newlands and Mendeleev's table?

  • weren't discovered then