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 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

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: Fe2+ Cl-

FeCl2

Fe3+ O2-

Fe2O3

Al3+ SO42-

Al2(SO4)3