Transcript 415CRISS CROSS

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: OXIDATION STATE( 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
Mg2+ Cl1• STEP THREE: cross the absolute values of charges as subscript of the
other element.
Mg2+
NOTE: the subscript
Cl1-
of 1 for Mg is
understood as in
algebra.
• STEP THREE continued
Mg1Cl2

MgCl2
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
Na1+ O2• STEP THREE: cross the absolute values of charges as subscript of the
other element.
Na1+
O2-
• STEP THREE continued
Na2O1
NOTE: the subscript
ALWAYS
WRITE
CATION(+
) FIRST

of 1 for O is
understood as in
algebra.
Na2O