C1.2.2 - Development of the Periodic table

• Problems with the early Periodic table

  Early periodic tables were incomplete, since many elements were unknown. Also, some elements were placed in groups with elements that were not similar to them.
• How did Mendeleev overcome these mistakes?

  He published his first periodic table of the elements in 1869. In it, he arranged the elements in order of increasing atomic weights. He also took into account the properties of the elements and their compounds. Mendeleev left gaps in his table for elements not known at the time. By looking at the properties of the elements next to a gap, he could also predict the properties of these undiscovered elements. 
• Compare Mendeleev's and modern Periodic table?

  Similarities: Metals and non-metals are placed separately in both periodic tables. The elements are arranged in groups and periods. Hydrogen is placed in the first row in both tables.

  Differences: In the Mendeleev's Periodic Table, elements were arranged on the basis of their atomic masses while in the Modern Periodic Table, elements were arranged on the basis of their atomic numbers.