Demonstrate through an activity how you write formula for chemical compounds. Write one page answer through an activity how would you write chemical formula for chemical compounds
Answers
Answer:
Ionic compounds do not exist as molecules. In the solid state, ionic compounds are in crystal lattice containing many ions each of the cation and anion. An ionic formula, like NaCl , is an empirical formula. This formula merely indicates that sodium chloride is made of an equal number of sodium and chloride ions. Sodium sulfide, another ionic compound, has the formula Na2S . This formula indicates that this compound is made up of twice as many sodium ions as sulfide ions. This section will teach you how to find the correct ratio of ions, so that you can write a correct formula.
If you know the name of a binary ionic compound, you can write its chemical formula. Start by writing the metal ion with its charge, followed by the nonmetal ion with its charge. Because the overall compound must be electrically neutral, decide how many of each ion is needed in order for the positive and negative charge to cancel each other out.
Example 5.4.1 : Aluminum Nitride and Lithium Oxide
Write the formula for aluminum nitride and lithium oxide.
Solution
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 5.4.2 : The Crisscross Method for lead (IV) oxide
Write the formula for lead (IV) oxide.
Solution
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 5.4.2
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 5.4.3 : Sulfur Compound
Write the formula for sodium combined with sulfur.
Solution
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 all subscripts that are 1.
Na2S
Exercise 5.4.3
Write the formula for each ionic compound.
sodium bromide
lithium chloride
magnesium oxide
Answer a:
Answer b:
Answer c:
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. For example, NO3− is the nitrate ion; it has one nitrogen atom and three oxygen atoms and an overall 1− charge. Table 5.4.1 lists the most common polyatomic ions.
Table 5.4.1 : Some Polyatomic Ions
ammonium ion
NH4+
acetate ion
C2H3O2− (also written CH3CO2−)
ionic
Exercise 5.4.6
Henry Agnew (UC Davis