Science 10 Unit 2

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

 e.g.: What is the formula for calcium oxide? Calcium is Ca 2+ Oxygen is O 2- 1 Ca 2+ ion and 1 O 2– ion CaO (remember, the subscript 1 is never written) Mg 2+ P 3- Mg 2+ P 3- Mg Mg 3 P 2

1. Write both symbols (+ion first) with ion charge 2. Criss-cross the charges and write them as subscripts 3. Reduce if possible MgO 2+2- (remember, the subscript 1 is never written) Mg O 22 Only use if you understand that the + and - charges must balance! Magnesium oxide

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

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 MgF 2 CuCl 2 Sodiumchloride Magnesiumfluoride copper has 2 ions: Cu 1+ Cu 2+ 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. CuCl therefore must be copper I chloride CuCl 2+1- CuCl 2 Therefore it is copper II chloride from periodic table Cl 1- 2-

Some groups of atoms act as individual ions e.g.SO 4 -2 Sulphate ion-2 OH -1 Hydroxide ion-1 PO 4 -3 Phosphate ion-3 When writing formulas and naming, pretend the polyatomic ion is just one atom and follow the rules as usual. e.g. K +1-3 K 3 PO 4 Potassium phosphate Don’t change the endings of polyatomic ions Ion Charge PO 4

Aluminum sulphate Copper (II) sulphate AlSO Al 2 (SO 4 ) 3 Cu +2 CuSO 4 Put brackets around the SO 4 to indicate 3 sulphate ions, not 43 oxygen atoms SO 4 -2 Don’t forget the roman numeral since copper has more than one combining capacity