Ternary Compounds
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Ternary Compounds

Ternary compounds are composed of three different elements.  The most common types of ternary compounds consist of a metallic cation (positive ion) and a polyatomic anion (negative ion).   The only common polyatomic ion with a positive charge is the ammonium ion.  At any rate, to write these formulas you will want to have reference tables .

Example 1.  Write the proper chemical formula for potassium hydroxide.

Step one - Write the symbols for the monatomic and polyatomic ions in the compound.  You will find the symbol potassium on table 1 (the periodic table).  Hydroxide is a polyatomic ion, which will be found on table 2 (polyatomic Chart).  Eventually you will recognize the name of a polyatomic ion, but for now if you can't find an ion on one table, look on the other.

Potassium = K      Hydroxide = OH

Step two - Look up the oxidation numbers of the ions involved (in table 5-2b and 5-2d, or some similar tables), and write them as superscripts to the right of the elemental symbols.

Potassium = K    Hydroxide = OH- 

Step three - Use the correct combination of ions to produce a compound with a net charge of zero. Parenthesis must be used if you need more than one of a polyatomic ion.  In this case, (+1) + (-1) = 0.  So, only one of each ion is used.  No subscripts are necessary.  If you needed more than one hydroxide ion, it would be put in parenthesis with the subscript on the outside.

KOH

Note the importance of upper and lower case

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Example 2.  Show the correct formula for Calcium Nitrate.

Step one - Write the symbols for the monatomic and polyatomic ions in the compound. 

Calcium = Ca       Nitrate = NO3

Step two - Look up the oxidation numbers of the ions involved (in table 5-2b and 5-2d, or some similar tables), and write them as superscripts to the right of the elemental symbols.

Calcium = Ca2+       Nitrate = NO3-

Step three - Use the correct combination of ions to produce a compound with a net charge of zero. Parenthesis must be used if you need more than one of a polyatomic ion.  In this case (+2) + 2(-1) = 0.   We need to show two nitrate ions in our formula.  The subscript is put on the outside of the parenthesis to show that the entire polyatomic ion is doubled.

Ca(NO3)2

The correct use of parenthesis will seem hard at first, but you must master this skill with practice!

 

 The Stock System

Some elements, like iron and lead, have more than one oxidation number.  If you were given a compound name like lead chloride, you would not know if you should used an oxidation number of +2 or +4 for the lead.  The stock system is used to specify which form of an element, that shows multiple oxidation numbers, is used in a particular compound.   A roman numeral is shown after the name of the positive ion (cation) to indicate the oxidation number of the positive ion.

Example 1.  Show the correct formula for lead (IV) nitrate.

Step one - Write the symbols for the  ions in the compound.

Lead = Pb           Nitrate = NO3

Step two - Look up the oxidation number of the negative ion involved (in table 5-2b and 5-2d, or some similar tables).  The positive ion will have a positive oxidation number equal to the roman numeral.  Write the numbers as superscripts to the right of the elemental symbols.

Lead = Pb4+          Nitrate = NO3-

Step three - Use the correct combination of ions to produce a compound with a net charge of zero. Parenthesis must be used if you need more than one of a polyatomic ion.

Pb(NO3)4   

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Example 2.  Show the correct formula for Copper (II) Fluoride

Step one - Write the symbols for the  ions in the compound.

Copper = Cu       Fluoride = F

Step two - Look up the oxidation number of the negative ion involved (in table 5-2b and 5-2d, or some similar tables).  The positive ion will have a positive oxidation number equal to the roman numeral.  Write the numbers as superscripts to the right of the elemental symbols.

Copper = Cu2+       Fluoride = F-

Step three - Use the correct combination of ions to produce a compound with a net charge of zero. Parenthesis must be used if you need more than one of a polyatomic ion.

CuF2

 

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