The periodic table

• Mendeleev arranged the 60 known elements at the time in order of atomic mass.

  He also lined up the elements in groups with similar properties.

  If the group properties did not fit, he swapped elements around and left gaps, assuming the atomic mass measurements were incorrect or elements were yet to be discovered.

  He even predicted the properties of the missing elements from group trends
• As well as having the same number of electrons in their outer shell, elements of the same group have the same number of electrons in each sub-shell.

  The similarity in electron configuration is what gives elements in the same group their similar chemistry.
• The new way of numbering groups in the periodic table is by numbering each column in the s-, d-, and p-blocks from 1-18. The old way numbered them from 1-7 and then 0 excluding the d-block.
• • Group 1= alkali metals
  • Group 2= alkaline earth metals
  • Groups 3-12= transition elements
  • Group 15 (5)= pnictogens
  • Group 16 (6)= chalcogens
  • Group 17 (7)= halogens
  • Group 18 (0)= noble gases