Writing Ionic Formulas

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Writing Ionic Formulas

• Chapter 19

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How to write a formula from the name.

• Write the element symbols given by the name
• Figure out the charge of each element.
• If the metal has only one possible charge, use
  the periodic table
• Group 1A is 1
• Group 2A is 2
• Al is 3
• Zn is 2
• Ag is 1
• Cd is 2

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How to write a formula from the name.

• If the metal is a transition metal, use the Roman
  Numeral as the charge
• Copper (II) is Cu2
• If the cation is a polyatomic ion, use the charge
  from your polyatomic ion sheet
• For the anion, use the periodic table or the
  polyatomic ion sheet
• Compound must be neutral, so all charges must
  cancel
• Use the crisscross method to find the subscript
• Then reduce to the lowest whole number fraction

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Crisscross method

• Charge from one atom becomes subscript of other
  atom
• For polyatomic ions, use parentheses to set the
  polyatomic ion apart from the subscript for that
  ion
• Reduce only the subscript that indicates how many
  of the ion there are, not the subscript that
  indicates what the ion is composed of.

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Crisscross method

• Examples
• Fe2 Cl-
• FeCl2
• Fe3 O2-
• Fe2O3
• Al3 SO42-
• Al2(SO4)3