formula of compounds with criss-cross method
Answers
Answer:
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.
...
Explanation:
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
Polyatomic Ions
Some ions consist of groups of atoms bonded together and have an overall electric charge. Because these ions contain more than one atom, they are called polyatomic ions. Polyatomic ions have characteristic formulas, names, and charges that should be memorized.
Some Polyatomic Ions
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−
The rule for constructing formulas for ionic compounds containing polyatomic ions is the same as for formulas containing monatomic (single-atom) ions: the positive and negative charges must balance. If more than one of a particular polyatomic ion is needed to balance the charge, the entire formula for the polyatomic ion must be enclosed in parentheses, and the numerical subscript is placed outside the parentheses. This is to show that the subscript applies to the entire polyatomic ion. An example is Ba(NO3)2.
Writing Formulas for Ionic Compounds Containing Polyatomic Ions
Writing a formula for ionic compounds containing polyatomic ions also involves the same steps as for a binary ionic compound. Write the symbol and charge of the cation followed by the symbol and charge of the anion.
Calcium Nitrate
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
Write the chemical formula for an ionic compound composed of the potassium ion and the sulfate ion.
Solution
Explanation Answer
Potassium ions have a charge of 1+, while sulfate ions have a charge of 2−. We will need two potassium ions to balance the charge on the sulfate ion, so the proper chemical formula is K2SO4. K2SO4