Answer:
A number of elements exist in a pure form in nature such as oxygen and nitrogen (in air), gold, silver, carbon (graphite and diamonds), and sulfur. A few very familar elements include: iron in steel, copper in pipes in your house or the copper coating on pennies, and aluminum popcans.
To find the ionic charge of an element you'll need to consult your Periodic Table. On the Periodic Table metals (found on the left of the table) will be positive. Non-metals (found on the right) will be negative. But you need to know the specific ionic charge elments.
To find the formula of an ionic compound, first identify the cation and write down its symbol and charge. Then, identify the anion and write down its symbol and charge. Finally, combine the two ions to form an electrically neutral compound.
Write the formulas for aluminum nitride and lithium oxide.
Solution
Write the formula for aluminum nitride Write the formula for lithium oxide
1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second. Al3+N3− Li+O2−
2. Use a multiplier to make the total charge of the cations and anions equal to each other.
total charge of cations = total charge of anions
1(3+) = 1(3-)
+3 = -3
total charge of cations = total charge of anions
2(1+) = 1(2-)
+2 = -2
3. Use the multipliers as subscript for each ion. Al1N1 Li2O1
4. Write the final formula. Leave out all charges and all subscripts that are 1. AlN Li2O
An alternative way to writing a correct formula for an ionic compound is to use the crisscross method. In this method, the numerical value of each of the ion charges is crossed over to become the subscript of the other ion. Signs of the charges are dropped.
Example 5.5.2 : The Crisscross Method for lead (IV) oxide
Write the formula for lead (IV) oxide.
Solution
Crisscross Method Write the formula for lead (IV) oxide
1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second. Pb4+O2−
2. Transpose only the number of the positive charge to become the subscript of the anion and the number only of the negative charge to become the subscript of the cation. The 4+ charge on Pb becomes the subscript of O and the 2- charge on O becomes the subscript of Pb.
3. Reduce to the lowest ratio. Pb2O4
4. Write the final formula. Leave out all subscripts that are 1. PbO2
Exercise 5.5.2
Write the chemical formula for an ionic compound composed of each pair of ions.
the calcium ion and the oxygen ion
the 2+ copper ion and the sulfur ion
the 1+ copper ion and the sulfur ion
Answer a:
Answer b:
Answer c:
Be aware that ionic compounds are empirical formulas and so must be written as the lowest ratio of the ions.
Example 5.5.3 : Sulfur Compound
Write the formula for sodium combined with sulfur.
Solution
Crisscross Method Write the formula for sodium combined with sulfur
1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second.
Na+S2−
2. Transpose only the number of the positive charge to become the subscript of the anion and the number only of the negative charge to become the subscript of the cation. The 1+ charge on Na becomes the subscript of S and the 2- charge on S becomes the subscript of Na.
3. Reduce to the lowest ratio. This step is not necessary.
4. Write the final formula. Leave out all subscripts that are 1. Na2S
Exercise 5.5.3
Write the formula for each ionic compound.
sodium bromide
lithium chloride
magnesium oxide
Answer a:
Answer b:
Answer c:
Explanation:
Name Formula
ammonium ion NH4+
acetate ion C2H3O2− (also written CH3CO2−)
carbonate ion CO32−
chromate ion CrO42−
dichromate ion Cr2O72−
hydrogen carbonate ion (bicarbonate ion) HCO3−
cyanide ion CN−
hydroxide ion OH−
nitrate ion NO3−
nitrite ion NO2−
permanganate ion MnO4−
phosphate ion PO43−
hydrogen phosphate ion HPO42−
dihydrogen phosphate ion H2PO4−
sulfate ion SO42−
hydrogen sulfate ion (bisulfate ion) HSO4−
sulfite ion SO32−
Write the formula for calcium nitrate.
Solution
Crisscross Method Write the formula for calcium nitrate
1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second. Ca2+NO−3
2. Transpose only the number of the positive charge to become the subscript of the anion and the number only of the negative charge to become the subscript of the cation. The 2+ charge on Ca becomes the subscript of NO3 and the 1- charge on NO3 becomes the subscript of Ca.
3. Reduce to the lowest ratio.
Ca1(NO3)2
4. Write the final formula. Leave out all subscripts that are 1. If there is only 1 of the polyatomic ion, leave off parentheses. Ca(NO3)2
