Naming Compounds Tutorial And Worksheet With Answers Page 3
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20Writing the Formulas of Ionic Compounds
Example: Write the formula for calcium bromide.
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.
Ca Br
2) Indicate the ratio of each ion in the compound using subscripts after each ion.
•
This step involves filling in the subscripts boxes as we did in the lecture:
Ca Br
•
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.
•
First, temporarily write the charge of each ion above the ion’s symbol.
2+
1-
Ca Br
•
Next, place numbers in the subscripts such that the total charge of the
compound is zero. Note that in this example, we need two bromide ions, each
has a charge of (1-) to cancel the (2+) charge of the calcium ion:
•
2(-1) + (+2) = 0 zero total charge.
2+
1-
Ca
Br
= CaBr
1
2
2
•
We saw a shortcut way to do this called the Criss-Cross Method (see your
chapter 3 notes)
2+
1-
Ca
Br
= CaBr
1
2
2
•
Note, we do not leave the charges written above the symbols in the
completed formula.
IMPORTANT: When there is more than one of a polyatomic ion in the formula, we use
parenthesis.
•
Not applicable in this example since there are no polyatomic ions in calcium
bromide.
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