Criss Cross-Charge-Empirical Formula OBJECTIVE: Too calculate the SUBSCRIPT MOLE RATIO of an ionic BINARY compound, GIVEN two elements in a compound. WHAT YOU NEED: Mass charges from periodic table. WHAT YOU MUST KNOW: the charges of the groups –Group 1 is 1+ –Group 2 is 2+ –Group 17 is 1- (in binary compounds, can vary in others.) –Group 16 is 2- (in binary, some exceptions not needed here) SKILL: the criss cross method. STEP#1: write the formula with charges for both atoms. STEP#2: the charge of one element becomes the subscript of the other, use absolute values of the charges. STEP#3: Reduce the subscripts to whole number integers (reduce them to lowest terms – empirical formula)

EXAMPLE: Mg and Cl STEP ONE: Cl is in group 17 and is 1- in binary compounds. Mg is in group 2 and is always 2+ in all compounds. STEP TWO: write formula with charges STEP THREE: cross the absolute values of charges as subscript of the other element. STEP THREE continued Mg 2+ Cl 1- Mg 2+ Cl 1- Mg 1 Cl 2 MgCl 2 NOTE: the subscript of 1 for Mg is understood as in algebra. 

EXAMPLE: Na and O STEP ONE: Na is in group 1 and is 1+ in binary compounds. O is in group 16 and is always 2- in all compounds(for now!). STEP TWO: write formula with charges STEP THREE: cross the absolute values of charges as subscript of the other element. STEP THREE continued Na 1+ O 2- Na 1+ O 2- Na 2 O 1 Na 2 O NOTE: the subscript of 1 for O is understood as in algebra. ALWAYS WRITE CATION(+ ) FIRST 