development of the periodic table

• in the early 1800's what were elements arranged in?


  arranged by atomic weight
• what did Mendeleev do to his periodic table?


  he left gaps in his finished periodic table as her knew that there was more elements to be found

  he also put the elements mainly in order of atomic weight but did switch the order of if the properties meant it should be changed
• what did the discovery of isotopes confirm?


  it confirmed that Mendeleev was correct not to put elements in a strict order of atomic weight but to also take account of their properties.
• what does the periodic table allow us to see?


  allows us to see the patterns in properties
• what are the elements now in order of?


  increasing atomic number
• what do elements with similar properties form on the periodic table?


  groups