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Counting atoms in a chemical fomula
• Add up the subscripts.
Ca3P2 = 5 atoms
• Don’t forget the “invisible” 1’s
CaCO3 = Ca1C1O3 = 5 atoms
• Multiply anything inside parentheses by the subscript on the outside.
Ba(NO3) = 1 Ba, 2 N, 6 O = 9 atoms22
Writing Chemical Formulas
• The metal (or + ion) is always written first.
• The nonmetal (or – ion) is always written last.
• The subscripts tell how many of each atom or polyatomic ion you have.
Writing Chemical Formulas
• EXAMPLE: Write the chemical formula for the ionic compound containing Al and Br.
#1 – determine the charges
Al +3
Br -1
Writing Chemical Formulas
• #2 – cross the charges. Do not carry the sign (+/-) and do not write the number 1.
Al 3
Br 1 #3 – simplify if
needed (lowest common factor)Not needed here
Writing Chemical Formulas
• USING POLYATOMIC IONSNa and OH-1
Na+1 OH-1 NaOH
Ca and PO4-3
Ca+2 PO4-3 Ca3PO42 ???
Writing Chemical Formulas
**If the polyatomic ion gets a subscript you MUST use parentheses.**
Ca3(PO4)2
Naming Ionic Compounds
With Main Group Metals1. Name the + Ion – Name
the metal.
2. Name the - Ion - Name the nonmetal, changing the ending to –ide