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Ionic compounds - EdexcelPolyatomic ions

When a metal element reacts with a non-metal element an ionic compound is formed. They have a giant lattice structure with strong ionic bonds. A lot of energy is needed to overcome these bonds.

Part ofChemistry (Single Science)Key concepts in chemistry

Polyatomic ions

Some are formed from groups of . The table shows the names and formulae of some .

CationsAnions
Ammonium, NH4+Hydroxide, OH-
Calcium, Ca2+Nitrate, NO3-
Sodium, Na+Carbonate, CO32-
Lead, Pb2+Sulfate, SO42-
CationsAmmonium, NH4+
AnionsHydroxide, OH-
CationsCalcium, Ca2+
AnionsNitrate, NO3-
CationsSodium, Na+
AnionsCarbonate, CO32-
CationsLead, Pb2+
AnionsSulfate, SO42-

The formulae of containing polyatomic ions are worked out in a similar way to single atom ions. Except when there is more than one polyatomic ion, then its formula is written inside brackets.

Example

Calcium hydroxide contains Ca2+ and OH- ions:

  • this is two positive charges and one negative charge
  • to balance it will need, one Ca2+ ions and two OH- ions
  • so the formula is Ca(OH)2

Question

Deduce the formula for sodium carbonate.

Question

Deduce the formula for lead nitrate.

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Naming ionic compounds with -ide and -ate
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Names and formulae of ionic compounds

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