Writing Ionic Formulas

How to write a formula from the name. Write the element symbols given by the name. The cation always goes first. 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+

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 Cu 2+ 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

Crisscross method Charge from one atom becomes subscript of other atom- do NOT write the sign when you write the number as a subscript 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.

Crisscross method Examples: Fe 2+ Cl - FeCl 2 Fe 3+ O 2- Fe 2 O 3 Al 3+ SO 4 2- Al 2 (SO 4 ) 3