Electronegativity trend to fluorine; drop-and-swap for ionic formulas (MgF₂)

  • Electronegativity trend runs along diagonals; fluorine is highly electronegative (arrow points to F).
  • In compounds, the written formula is the smallest whole-number ratio (empirical formula), not the exact counts in a sample (e.g., 100 Mg and 200 F would still be written as MgF₂).
  • Drop and swap method for ionic compounds:
    • Identify ion charges for the ions involved (cation and anion).
    • Swap the ions to balance charges and use the smallest whole-number ratio for subscripts.
  • Example: magnesium fluoride
    • Ions: extMg2+,extF−ext{Mg}^{2+}, ext{F}^{-}
    • Ratio: 1:21:2 (to balance +2 with two extF−ext{F}^{-})
    • Resulting empirical formula: extMgF2ext{MgF}_{2}
  • General takeaway: drop and swap yields the empirical formula by balancing charges to achieve a neutral compound.