WRITING CHEMICAL FORMULAS AND NAMES
INORGANIC NOMENCLATURE AND FORMULA WRITING
There are two methods to name inorganic compounds and write their chemical formulas:
To decide which method is suitable, it is first necessary to decide what class of compound is being named: ionic or covalent. Recall that ionic compounds are made up of a metal and a nonmetal, where one of more electrons has been effectively transferred from one atom to another. For example, salts, such as sodium chloride or potassium sulfide, are ionic compounds. Covalent compounds are made up of two nonmetals; examples are carbon dioxide and phosphorus pentachloride. Covalent compounds are characterized by covalent bonds, in which electrons are shared between two atoms. If the sharing is unequal, the bond is said to be polar- covalent, and one end of the molecule has a partial negative charge. Such molecules are referred to as dipolar and have dipole moments, which are quantitative measures of their polarity. [Higher order polarities, like quad- rupoles (e.g., CO2) and hexadecapoles (e.g., SF6) do exist and can be calculated but are usually small and make an insignificant contribution to the net polarity.] If the sharing is equal, the bond is said to be nonpolar covalent; the molecule has no charge separation and hence no positive or negative end.
If the compound in question is ionic, use the crisscross method of nomenclature. Write the symbols of metal and nonmetal elements side by side (recall that a nonmetal ion may be a single anion or a polyatomic oxoanion, such as sulfate or nitrate), along with their respective valences or oxidation states as right-hand superscripts. Then crisscross the superscripts, that is, interchange them for the two elements, and write them as subscripts, omitting the plus or minus signs. If the two subscripts are the same, drop them both. In nomenclature, the name of the metal element is given first and that of the nonmetal element second, changing it to an “ide” ending, such as chloride or sulfide, or using the name of the oxoan- ion, like sulfate or nitrate.
If a compound is covalent, the Greek prefix method is used. Here, knowledge of valences is unnecessary, but it is necessary to know the first 12 numbers in Greek: mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca, undeca, and dodeca. The proper Greek prefix then precedes each element in the formula. As above, the name of the second element takes on the suffix “-ide.”
It should be remembered that many compounds have chemical names as well as common names. A compound generally has only one correct chemical name but may have more than one common name. For example, the compound Ca(OH)2 has the chemical name calcium hydroxide; it may also be referred to commonly as slaked lime or hydrated lime.
Table 1.1. Names and symbols of simple cations and anions (Type I), cations with multiple oxidation states (Type II), and polyatomic/oxoanions
ClO2 chlorite ion
Note: The following halogens can form polyatomic ions in the same manner as chlorine:
BrO- hypobromite ion IO- hypoiodite ion ВЮ2- bromite ion IO2- iodite ion
ВгОз- bromate ion IO3- iodate ion
BrOzf perbromate ion IO4- periodate ion
Write the correct chemical formula for each of the following compounds:
A. Calcium chloride
B. Aluminum sulfate
C. Iron (III) phosphate
D. Sulfur trioxide
E. Diphosphorus pentoxide
Compounds A to C are comprised of a metal and a nonmetal and are therefore ionic, so the crisscross method applies.
Compounds D and E are comprised of two nonmetals and are therefore covalent, so the Greek prefix method applies.
A. Calcium chloride: Ca2+ and Cl- are the two ions (note that the numeral 1 in front of the minus sign in the chloride ion has been omitted because 1’s are understood and not written).
Crisscrossing the superscripts 2 and 1 while omitting the signs gives CaCl2.
Note that the subscript 1 next to Ca has been omitted because 1’s are understood and are not written when specifying numbers of atoms.
B. Aluminum sulfate: Al3+ and (SO4)2- are the two ions.
Crisscrossing gives Ah(SO4)3.
Note that parentheses are necessary to express the fact that the subscript 3 refers to and thus multiplies both the sulfur and oxygen atoms (subscripts) inside the parentheses.
C. Iron (III) phosphate: Fe3+ and (PO4)3- are the two ions.
Note that the iron (III) notation refers to the Fe3+ species, to be distinguished from the Fe2+ species, written as iron (II). This is the modern way of distinguishing the ferric ion from the ferrous ion. This same system of notation, using Roman numerals in parentheses, is used for all transition metals with multiple oxidation states.
Crisscrossing gives Fe3(PO4)3.
Note that fine-tuning is necessary in this case. Since both subscripts are 3, they are omitted. Furthermore, the parentheses then become unnecessary and are dropped.
The fine-tuned answer is FePO4.
D. Sulfur trioxide: S and O are the elements.
Sulfur is “mono” or 1 (understood and not written), and oxygen is “tri” or 3. Hence the formula is SO3.
E. Diphosphorus pentoxide: P and O are the elements.
Phosphorus is “di” or 2, and oxygen is “penta” or 5. Hence the chemical formula is P2O5.
Find the total number of atoms in one formula unit of compounds A, B, and C in Example 1.2.
In each case, the subscripts next to all the atoms in the formula unit are added together. Where parentheses are used, subscripts inside parentheses must first be multiplied by subscripts outside parentheses.
For CaCf: 1 Ca atom + 2 Cl atoms = 3 atoms total
For Ab(SO4)3: 2 Al atoms + 3 S atoms + 12 O atoms = 17 atoms total
For FePO4: 1 Fe atom + 1 P atom + 4 O atoms = 6 atoms total
Note: The percent oxygen (O) by number in Ak(SO4)3 is 12/17 x 100 or 70.6%, while in FePO4, it is 4/6 x 100 or 66.7%. Compare this calculation to the calculation for percent by mass in Example 1.6.