Naming Compounds Tutorial And Worksheet With Answers Page 5
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20Examples: Writing the Formulas of Ionic Compounds
Write the formula for iron(II) phosphate.
1) Write the symbol/formula of the first ion in the compound’s name, then the
symbol/formula of the second ion in the compound’s name.
•
When you see a polyatomic ion (nitrate), look up the formula and charge
in the table of polyatomic ions.
Fe PO
4
2) Indicate the ratio of each ion in the compound using subscripts after each ion.
b. This step involves filling in the subscripts boxes as we did in the lecture:
Fe PO
4
•
The ratio of the ions is deduced by balancing the charges of the ions.
•
This is done so that the total charge in the crystal, when large numbers of
cations and anions combine, is equal to zero.
•
We find the ion’s charge from its position on the periodic table or we
look it up in a table in the case of polyatomic ions.
•
Transition metal with varying charges will be written in the compound
name in Roman numerals.
•
In this example, now we know the charge on the Fe ion is 2+
•
First, temporarily write the charge of each ion above the ion’s symbol.
2+
3-
Fe PO
4
•
Next, place numbers in the subscripts such that the total charge of the
compound is zero. Note that in this example, we need two phosphate ions, each
2+
has a charge of (3-) and three Fe
ions to balance the charge:
•
2(-3) + 3(-2) = 0 zero total charge.
2+
3-
Fe
PO
3
4 2
•
We saw a shortcut way to do this called the Criss-Cross Method (see your
chapter 3 notes)
2+
3-
Fe
PO
3
4 2
IMPORTANT:
unit we use
When there is more than one of a polyatomic ion in the formula
parenthesis. There are 2 ions of phosphate in iron(II)phosphate.
Fe
PO
= Fe
(PO
)
3
4 2
3
4
2
5
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