Polyatomic Ions and Compounds

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Polyatomic Ions and Compounds
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• Polyatomic Ion
• A group of atoms that tend to stay together and
  carry an overall ionic charge.

1 -1 -2 -3
NH4 (ammonium)  OH- (hydroxide)  CO32- (carbonate)  PO43- (phosphate) 
  NO3- (nitrate)  SO42- (sulfate)   
  ClO3- (chlorate)  CrO42- (chromate)   
  HCO3- (hydrogen carbonate or bicarbonate)  Cr2O72- (dichromate)   
  MnO4- permanganate)     
   
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Writing Formulas for Polyatomic Compounds

• To write a chemical formula for a polyatomic
  compound, follow these steps
• 1. Write the symbols for each substance, with
  the positive first.
• 2. Write the ionic charge above each symbol.
• 3. Add more of the positive or negative ions
  until the total charges are balanced.
• 4. Write the formula using subscripts to
  indicate the number of ions of each type.

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For Example Sodium Sulfate
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Example 2 Calcium Hydroxide
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Naming Polyatomic Compounds

• To name a polyatomic compound,
• 1. The positive ion is named first
• 2. The negative ion is named second. Do not
  change the ending of a polyatomic ion.

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For example

• 1. Sodium and phosphate
• 2. Na CO3
• 3. K SO4

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Multi Valent Nomenclature.

• Some transition metals have more than one valence
  option.
• Ex. Sn, Au, Pb, Fe.
• For determining the name you need to first look
  at what it is paired with.
• Ex. Fe2O3
• Oxygen has a ionic charge of -2 so in total there
  is -6 as a charge for oxygen (3 x -2). Iron needs
  to match so 2 x (3) will match it. Thus the
  charge for iron MUST be 3
• Then we write the name with roman numerals
• Iron(III)oxide

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Practice Questions

• Pg 151 1-8

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Multi Valent Contd

• When you are given the name you read the roman
  numerals to get the ionic charge of the element
• Ex. Nickel (II) phosphate
• Ni has a charge of 2 according to the name
• Phosphate has a charge of (-3)
• The formula would be Ni3(PO4)2
• REMEMBERPolyatomic Ions are treated as a
  unit. A subscript is put outside brackets to
  have a consistent charge