6.4 Writing formulas for ionic compounds

ide The names of all ionic compounds which are made of two elements always end in –ide NaCl is sodium chloride CaCL 2 is calcium chloride AlCl 3 is aluminum chloride

Binary ionic compounds Compounds composed of two elements are called binary compounds Compounds composed of a metal and a non- metal are called ionic compounds: NaCl CaF 2 K 2 S So these are all binary ionic compounds!

One metal ion from over here and one non-metal ion from over here make up a binary ionic compound

Balancing the charges When writing the formula for any binary ionic compound, the charge of the cation(s) must equal the charge of the anion(s) Using the periodic chart, the charges of the ions formed from elements in Group A are easily determined

Remember where the Group A elements are?

Calcium bromide Calcium is in Group 2A, so it loses two electrons and becomes Ca +2 Bromine is in Group 7A, so it gains an electron and becomes Br - To balance the charges when these two ions form an ionic compound, we must have two bromines for each calcium We write this as CaBr 2 Binary ionic compounds end in “ide”

Balance these binary ionic formulas Na + O Mg + S Ca + F Al + Br Ba + O

Criss-cross method If you know the charges of the two ions, you can use the charge of the first ion as the subscript of the second, and vice versa: Fe +3 and O -2 Fe +3 O -2 Fe 2 O 3

Naming ionic compounds What if you want to write the name of the formula instead? You will need to include all the information that the formula does This is easy for binary ionic compounds involving Group A ions – NaCl is sodium chloride – MgBr 2 is magnesium bromide

When writing the names of ionic compounds involving transition elements (Group B), it is necessary to look at the anion to determine the charge of the cation. (transition or Group B elements can have several oxidation states)

Remember where the Group B elements are?

copper oxide can mean either CuO or Cu 2 O, depending on which copper cation is involved. Since we know the oxygen ion always has a charge of -2, we also must include the +2 charge when writing the name for CuO, and the +1 charge when writing the name for Cu 2 O. CuO is written copper(II) oxide Cu 2 O is written copper(I) oxide

Since oxygen always has a charge of -2, if you were writing the name for SnO 2, you must assume that Sn (tin) has a charge of +4 because of the two oxygens in the formula. Therefore, the correctly written name of the substance is tin(IV) oxide

Write the names of these formulas Fe 2 O 3 Ni 2 O 3 MnO 2 Cu 2 S FeCl 3

Ternary ionic compounds Ternary compounds are compounds which contain three different elements. Ternary ionic compounds usually contain a polyatomic ion. Remember, polyatomic ions are treated as a unit and have a specific charge.

Ternary ionic compounds (three elements) Use the same method (criss-cross) of balancing a ternary compound as you would a binary compound K +1 and (SO 4 ) -2 K +1 (SO 4 ) -2 K 2 SO 4