Writing ionic compounds (charges)

• Formula writing for ionic compounds-

  Binary ionic compounds

  • ionic bond between two atoms
  • total charge of all atoms must equal zero

  identify the charges of both + and - ions

  • if charges are equal no subscripts are needed
  • if charges don’t equal zero do the criss cross method

  example: Ca^2+ and O^2- when added they both equal zero so no need for extra steps or subscripts—> CaO

  however with Na^1+ and O^2- they don’t equal zero so you have to simplify it by doing the criss cross method. You would get Na2O.
• Criss cross method

  
• Writing ternary ionic compounds

  Polyatomic ions

  • combinations of 2 or more non metals that form common ions
  • most are negatively charged except NH4^1+
  • Must be placed in () if more than two are needed. the charges and amount of the compound are outside

  Ternary Ionic Compounds

  • ionic bond between an element and a polyatomic ion
• Cross cross method with ternary ionic

  example: Na and (PO4)3- would simplify to Na3PO4. No parenthesis needed because there is only one phosphate