Naming Compounds Tutorial And Worksheet With Answers Page 6
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20Examples: Writing the Formulas of Ionic Compounds
Write the formula for barium sulfide.
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.
Ba S
2) Indicate the ration 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:
Ba S
•
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+
2-
Ba S
•
Next, place numbers in the subscripts such that the total charge of the
compound is zero. Note that in this example, we need one sulfide ion, with a
charge of (2-) to cancel the (2+) charge of the barium ion:
•
(-2) + (+2) = 0 zero total charge.
2+
2-
Ba
S
= BaS
1
1
•
We saw a shortcut way to do this called the Criss-Cross Method (see your
chapter 3 notes)
2+
2-
Ba
S
= BaS
2
2
•
Note, the subscripts in ionic compound represent the ratio in which large
numbers of anions and cations combine to form the ionic compounds.
Since we want the lowest ratio: we use 1:1, since 2:2 = 1:1
Ba
S
= BaS
2
2
6
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