Formula Writing

Writing formulas for binary ionic compounds

  • Binary compounds- composed of 2 elements
  • Cross method – the numerical charge of each ion is crossed over and used as a subscript for the other ion.
  • Ex) Ca 2+          S2- → Ca2S2 → CaS
  • Keep in mind, the charges are written as superscripts and the values in the compound are written as subscripts, could not format it correctly.
  • This compound is called Calcium   Sulfide
  • Remember formulas for ionic compounds show the lowest, whole number ratio of the ions.

Writing formulas for ternary ionic compounds

  • Ternary compounds- contain atoms of 3 different elements (usually polyatomic ion)
  • Same method using charge of polyatomic ion
  • Use parentheses around every polyatomic ion
  • Ex) Ca2+         (CO3 2-)  → Ca2(CO3)2 → CaCO3
  • This compound is called Calcium     Carbonate 

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