Naming Ionic Compounds

Lowest common multiple of 2 and 3 is 6 1. Writing Formulas In an ionic compound, positive charges must balance the negative charges. e.g.: What is the formula for magnesium phosphide? Magnesium is Mg2+ Phosphorous is P3- Lowest common multiple of 2 and 3 is 6 3 Mg2+ ions and 2 P3– ions Mg3P2

Mg3P2 CaO Mg2+ P3- Mg2+ P3- Mg2+ 6+ 6- e.g.: What is the formula for calcium oxide? Calcium is Ca2+ Oxygen is O2- 1 Ca2+ ion and 1 O2– ion CaO (remember, the subscript 1 is never written)

Only use if you understand that the + and - charges must balance! Short-cut Only use if you understand that the + and - charges must balance! Magnesium oxide 1. Write both symbols (+ion first) with ion charge 2+ 2- Mg O 2. Criss-cross the charges and write them as subscripts Mg O 2 2 MgO 3. Reduce if possible (remember, the subscript 1 is never written)

Sometimes, metal ions have more than one combining capacity e.g. Cu2+ Cu1+ Fe3+ Fe2+ The name of the compound will include a roman numeral that tells you which combining capacity to use 1+ I 5+ V 2+ II 6+ VI 3+ III 7+ VII 4+ IV e.g. Iron II oxide Iron III oxide 2+ 2- 3+ 2- Fe O Fe O FeO Fe2O3

2. Naming Cu Cu 1. Write the name of the metal first. 2. Write the name of the non-metal second, and change the ending to “-ide” 3. ONLY IF THE METAL HAS 2 OR MORE ION CHARGES, Include a roman numeral to indicate which one it is. e.g. NaCl Sodium chloride MgF2 Magnesium fluoride 1+ Cu CuCl2 copper has 2 ions: 2+ Cu which one is it?

To find out which copper ion it is, look at the charge on the anion and balance the charges in reverse. CuCl2 2+ 1- Cu Cl from periodic table 1- Cl 2- Therefore it is copper II chloride CuCl therefore must be copper I chloride

Polyatomic Ions K PO4 K3PO4 Some groups of atoms act as individual ions Ion Charge e.g. SO4-2 Sulphate ion -2 OH-1 Hydroxide ion -1 PO4-3 Phosphate ion -3 When writing formulas and naming, pretend the polyatomic ion is just one atom and follow the rules as usual. +1 -3 e.g. K PO4 Potassium phosphate K3PO4 Don’t change the endings of polyatomic ions

More Examples! Al SO4 Al2(SO4)3 Cu SO4 CuSO4 Aluminum sulphate +3 -2 Al SO4 Al2(SO4)3 Aluminum sulphate Put brackets around the SO4 to indicate 3 sulphate ions, not 43 oxygen atoms +2 -2 Cu SO4 CuSO4 Copper (II) sulphate Don’t forget the roman numeral since copper has more than one combining capacity