Ion Charge and the Formulas of Ionic Compounds

Recall Compounds are formed when each atom in the compound attempts to have a ‘full’ outer shell of valence electrons E.g. NaCl

Ionic Compounds Form between a metal and a nonmetal

Combining capacity (Ion Charge) Located on the top right hand corner of an elements box on the periodic table. Definition – The ability of an element to combine with another element. Related to its ability to donate and accept electrons

For metals the combining capacity is usually positive and for non-metals it is negative. When ionic compounds form, every electron that is given up by a metal atom must be accepted by a non-metal atom. If the elements have equal but opposite ion charges, then they will combine in the ratio 1:1. Ex. NaCl

What about Al3+ S2- If the two elements have unequal and opposite ion charges, then the elements will combine in a ratio so that the total number of electrons transferred equals the total number of electrons accepted.

Example – Aluminum sulfide Aluminum – Al3+ Combining Capacity = +3 Sulfur– S2- -2 Ratio of combining capacities = 3 sulfurs for every 2 aluminums.

Short Cut - Criss –Cross (Drop and Swap) Method Example: Putting aluminum and sulfur together (aluminum sulfide) 1. Identify each element and its combining capacity. Ex. Aluminum sulfide= Al3+ S2- 2. Drop and swap the numbers from the combining capacity (Criss-cross the combining capacity) . Ex. Al3+ S2- 3. Put the symbols together and get rid of the charge values. Al2S3

Al2S3 (Notice that 2Al3+ = 6+ and 3S2- = 6-; thus, compound charge is neutral (0))

Putting calcium and oxygen together (calcium oxide). Identify each element and its combining capacity. Ex. Ca2+ and O2-. Criss cross charges. (Drop and Swap) Ex. Put the symbols together, get rid of the charge symbols and values. Ca2O2 Reduce to the lowest common multiple (note-you do not write the one) CaO

What about Polyatomic Ions? Do the same thing! Remember to make sure that the number of polyatomic ions when swapped is outside the brackets Ex. Aluminum and sulfate

Practice Write the formulas for the compounds formed in each of the following: (a) silver and sulfur (b) magnesium and chlorine (c) zinc and bromine (d) calcium and nitrogen (e) calcium and nitrate

Multivalent Many elements are multivalent (they have more than one charge) For these, you will see a Roman numeral to tell you which to use. Ex. Iron (II) means to use 2+ Manganese (III) means to use 3+ See page 173 (c) McGraw Hill Ryerson 2007

Try these: (f) cobalt(III) and oxygen (g) copper(I) and nitrate