Periodic trends

• Why Does Electronegativity Vary Down Groups 13–17?
  Electronegativity usually falls down group 17, while groups 13–16 show irregularities from d- and f-block contraction; compare Pauling values.
• Which Is Stronger: HBrO4 or HIO4, and HBr or HI?
  HBrO4 is stronger than HIO4, but HI is stronger than HBr. The opposite trends arise from oxyanion stabilization and weakening hydrogen–halogen bonds.