Ionic Compounds: Writing Formulas

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Ionic CompoundsWriting Formulas
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Empirical Formulas

• smallest whole-number ratio of elements
• Ionic compounds empirical formulas only

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Identify the empirical formulas

• H2O
• H2O2
• CaF2
• NaCl
• CH4
• C2H6
• FeCl3

Empirical
Molecular
Empirical (Ionic)
Empirical (Ionic)
Empirical
Molecular
Empirical (Ionic)
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Binary Ionic Compounds

• Composed of two different elements
• Positive monatomic metal ion
• Negative monatomic nonmetal ion

Note binary compound may contain more than 2
ions but only 2 kinds of ions like Al2O3
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Oxidation Number

• Charge of a monatomic ion
• Some elements form only 1 ion
• Some elements can form more than 1 ion

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Oxidation State of Metal Elements

• Group 1 ions always 1
• Group 2 ions always 2
• MOST group 13 ions are 3
• Groups 14, 15, 16, and 17 multiple oxidation
  states
• Transition metals (Group B) can have more than 1
  oxidation state

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Writing Formulas Binary Ionic Compounds

• first rule is
• POSITIVE ION FIRST
• (metal)

How do you get subscripts?
start by writing the ions
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• Compounds are neutral
• Sum of all charges must 0
• () and (-) charges must each other

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Charges Equal but Opposite

• Na1 and Cl-1 NaCl 1 (-1) 0
• Mg2 and O-2 MgO 2 (-2) 0
• Al3 and P-3 AlP 3 (-3) 0
• Rule write the symbols, () FIRST!

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Try a few formulas

• Li1 and I-1
• Ca2 and O-2
• Al3 and N-3
• K1 and F-1
• Ba2 and S-2

LiI
CaO
AlN
KF
BaS
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Charges Opposite but NOT equal

• Criss-Cross Method
• Mg2 and Cl-1, CROSS and DROP!
• ( only, forget
  signs!)

• Mg1Cl2 (if subscript is 1, forget it)

• MgCl2 (1 Mg2 and 2 Cl-1)
• check math 2 -1 -1 0

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Try a few formulas

• Ca2 Cl-1
• Na1 O-2
• Cs1 S-2
• Al3 Cl-1
• Al3 Se-2
• Mg2 F-1
• K1 N-3

CaCl2
Na2O
Cs2S
AlCl3
Al2Se3
MgF2
K3N