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Binary Ionic Compounds

A binary compound is one that is composed of two elements that have been chemically combined. An ionic compound is one formed when a metal chemically combines with a non-metal. This chemical combination will always result in an ionic compound. In determining the formula of a binary ionic compound we will follow a set of steps until you are comfortable with the process. You will need to know or at least have access to a listing of common ions, their symbols, their charges (valences), and their names. Some elements have more than one possible charge or valence. These are referred to as the multi-valent elements. Iron for instance has a +2 charge (Iron II or Ferrous) in some compounds and a +3 charge (Iron III or Ferric) in other compounds so we will have to recognize in which state the Iron is in and write the formula accordingly.

• Mercury Hg⁺ (Mercury I) or Mercurous and Hg⁺² (Mercury II) or Mercuric
• Copper Cu⁺ (Copper I) or Cuprous and Cu⁺² (Copper II) or Cuprous
• Lead Pb⁺² (Lead II) or Plumbous and Pb⁺⁴ (Lead IV) or Plumbic
• Tin Sn⁺² (Tin II) or Stannous and Sn⁺⁴ (Tin IV) or Stannic
• Cobalt Co⁺² (Cobalt II or Cobaltous) and Co⁺³ (Cobalt III or Cobaltic)

All Group 1 elements in the Periodic Table are +1 in compounds. All Group 2 elements in the Periodic Table are +2 in compounds.

It is best to get a more complete listing of these common ions and their charges, symbols, and names.

The procedure that can be followed when confronted with the name of a compound and you wish to write its formula is as follows:

1. Identify the symbol of the cation (first part of the name) and the anion
2. Identify the valence or charge of each symbol and place it in parenthesis just above the symbol
3. Balance the total positive and negative charge on the cation and anion. You ask yourself do the total positive charge and total negative charge add up to zero. If the answer is no then we ask how many of each ion must we have in order to balance charge. We must have the same number of positive charges as we do of negative charges. Another way of saying that is that they must add up to zero.
4. Once you have determined the number of units of the cation and anion those become the subscripts which are placed right after the respective symbol.

    

So for example what is the formula of Copper (I) Oxide?

1. Identify the symbols of the cation and anion

   Copper is Cu and Oxide is O

2. Identify the charge for each and place above the symbol in parenthesis

   For Copper I that would be +1 and for Oxide that would be -2

3. Balance the positive and negative charges

   Since each Copper is +1 and each Oxide is -2 then it will take two Cu⁺ to balance one oxide with a -2 so that 

   2(+1) + 1(-2) = 0. The numbers outside the parenthesis become the subscripts in the formula

4. Write the formula placing the subscripts right after the symbol they go with.

   Cu₂O

Also note that all binary compounds end in "ide"

Let's try another:

What is the formula of Calcium Nitride?

1. Identify the symbols of each part of the name

   Calcium symbol is Ca and Nitride symbol is N

    

2. Identify the charge for each

   Calcium belongs from Group 2 which always has a +2 and Nitride will be a single Nitrogen with a -3 charge

    

3. Balance charge

   Since Calcium is +2 and Nitride is -3 the only way to balance them is to have three Calciums and two nitrides

    

4. Write the symbol beginning with the symbol that is first in the name and include the subscript after each symbol

   Ca₃N₂

   Ternary Ionic Compounds

A Ternary compound is one that has three or more elements involved in the compound. These ionic compounds will have one or more polyatomic ions. These are ions that have two or more elements clustered together with a charge assigned to the cluster. Some examples are the Carbonate ion (CO₃^-2), Phosphate (PO₄^-3), Acetate (C₂H₃O₂^-), and the Chromate (CrO₄^-2). There are many more mostly negative ions. These should be on your list of common ions. The rules for writing the formulas of ternary ionic compounds are pretty much the same as for binary compounds except you will be using polyatomic ion names. One other thing to keep in mind is that if you have more than one polyatomic ion to indicate in the formula, you group the polyatomic ion is a set of parenthesis and then place the number of those ions on the outside of the parenthesis as a subscript. If you have only one polyatomic ion to indicate in the formula you do not use the parenthesis.

What is the formula for Iron (III) Carbonate?

1. Identify the symbol of the cation (first part of the name) and the anion

   The symbol for Iron is Fe and the symbol for Carbonate which is a polyatomic ion is CO₃

2. Identify the valence or charge of each symbol and place it in parenthesis just above the symbol

   The valence for Iron (III) is +3 and the valence for Carbonate is -2

3. Balance the total positive and negative charge on the cation and anion. You ask yourself do the total positive charge and total negative charge add up to zero. If the answer is no then we ask how many of each ion must we have in order to balance charge. We must have the same number of positive charges as we do of negative charges. Another way of saying that is that they must add up to zero. Since an Iron (III) has a +3 charge and the Carbonate ion has a -2 then it would take two Fe⁺³ units to balance three CO₃^-2 units

4. Once you have determined the number of units of the cation and anion those become the subscripts which are placed right after the respective symbol.

   Fe₂(CO₃)₃

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