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Ions and ionic formulae
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You need to know the charges of common ions and use them to write formulae.
Ions from the Periodic Table
Metals lose electrons to form positive ions; non-metals gain electrons to form negative ions.
Other ions to learn
Writing formulae
The total positive charge must equal the total negative charge.
Al3+ and O2−: 2 × 3+ = 6+, 3 × 2− = 6−, so Al2O3.
Use brackets for more than one compound ion: Ca2+ and OH− give Ca(OH)2; NH4+ and SO42− give (NH4)2SO4.
Al3+ and O2−: 2 × 3+ = 6+, 3 × 2− = 6−, so Al2O3.
Use brackets for more than one compound ion: Ca2+ and OH− give Ca(OH)2; NH4+ and SO42− give (NH4)2SO4.
Write the formula of iron(III) sulfate.
- Fe3+ and SO42−
- Lowest common multiple of 3 and 2 is 6.
- 2 Fe3+ (6+) and 3 SO42− (6−)
Answer: Fe2(SO4)3
Groups 1, 2, 3 form 1+, 2+, 3+ ions; groups 5, 6, 7 form 3−, 2−, 1− ions. Learn Ag+, Cu2+, Fe2+, Fe3+, Pb2+, Zn2+, H+, NH4+, OH−, NO3−, CO32−, SO42−. Balance the charges and use brackets for compound ions.
The interactive lesson includes the diagrams for this topic.
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