Transcript Writing Chemical Formulas

Writing Chemical
Formulas
Fe2O3 or FeO???
Formulas are Neutral
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In order to write the chemical formula you
must know the oxidation number
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To be neutral, charges must balance!
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Al has a +3 oxidation number
Cl has a –1 oxidation number
+3-1-1-1 = 0
It takes 3 Cl atoms to balance one Al atom!
AlCl3 is the correct chemical formula
Criss-Cross Method
Fe+3
O-2
= Fe2O3
Criss-cross only the numbers!!!!
C+4
Cl-
= CCl4
It works every time!!!
Simplify When Needed
Mg+2
O-2
= Mg2O2 = MgO
Simplify!!
Polyatomics
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You must use parentheses when dealing
with polyatomics
Fe+3
OH- = Fe(OH)3
Criss-cross still works!!!!