Observations in Science
2.7 Chemical Nomenclature
is described by replacing the ending from its name with the suffi x -ide. Both elements will be preceded by a number-designating prefi x except that when there is only one atom of the fi rst element, it does not carry the prefi x mono-. An example of this procedure is provided in Example Problem 2.5.
E X A M P L E P RO B L E M 2 . 5
What are the systematic names of the following compounds? (a) N2O5, (b) PCl3, (c) P4O6
Strategy The fi rst element listed retains its full name and adds a prefi x when more than one atom of it is in the compound. The second element will retain only the root of its name, followed by -ide, and it too takes a prefi x to indicate the number of atoms.
Solution (a) N2O5: dinitrogen pentoxide, (b) PCl3: phosphorus trichloride (remem- ber: this is not called monophosphorus trichloride), (c) P4O6: tetraphosphorus hexoxide.
(The a in hexa- is dropped here to simplify pronunciation.)
Check Your Understanding What are the names of the following compounds?
(a) CS2, (b) SF6, (c) Cl2O7
Naming Ionic Compounds
The iron chlorides shown earlier in Figure 2.16 are examples of binary ionic com- pounds. Because ionic compounds must be neutral, the positive and negative charges of the ions must balance each other and only one formula unit is possible. Therefore, once one of the charges is specifi ed in the name, the entire formula is known. The rules of nomenclature specify that the positively charged species, the cation, be speci- fi ed with enough information about its charge to indicate the complete formula. Un- fortunately for novices in chemistry, some groups of the periodic table have only one possible cationic charge, whereas other groups (such as Group 8 that contains iron) have more than one possibility.
The most common cations are summarized in Table 2.5. Note that the cations of all Group 1 metals have a charge of 1+ and those of Group 2 have a charge of 2+. Because the nomenclature system was designed by chemists to be used by chemists, it assumes that we know this type of factual information. The nomenclature system gives
(a) Dinitrogen oxide, N2O
(d) Dinitrogen trioxide, N2O3
(e) Dinitrogen tetroxide, N2O4
(f) Dinitrogen pentoxide, N2O5 (b) Nitrogen
monoxide, NO
(c) Nitrogen dioxide, NO2
Nitrogen forms a number of binary compounds with oxygen.
no indication of charge for groups that always have the same charge. When we look at the charges in transition metals, however, we note that they often form two cations and some even more than two. When we encounter such a situation, we indicate the charge of the cation in the name using a Roman numeral in parentheses following the elemental name. Thus Fe2+ has the name iron(II), and Fe3+ has the name iron(III).
An older system was once used that distinguished these two ions by using a suffi x.
The smaller charge formed a name ending in -ous (ferrous for Fe2+ in the case of iron whose root, ferr, comes from the Latin word for iron), and the larger charge ended in -ic (ferric for Fe3+). We will not use this system in this book, but you may see these older names on some laboratory chemicals.
We’ve already seen some examples of elements that form monatomic anions. The most common are halogens. There are also several polyatomic anions that we will en- counter often in this book. Those listed in Table 2.6 are the most common anions that we will see throughout the text.
The naming convention for monatomic ions is familiar from the discussion of covalent molecules above: the name of a monatomic anion is the root of the ele- ment name with the suffi x -ide added. Thus Cl− is chloride, Br− is bromide, and so on. We can now name FeCl2 and FeCl3 as iron(II) chloride and iron(III) chloride, respectively.
Many compounds contain polyatomic anions, including those shown in Table 2.6.
Most often, the names of these polyatomic ions are memorized rather than being obtained by a systematic nomenclature rule. There is, however, a system for poly- atomic anions that contain oxygen and one other element, oxyanions. The base name of the oxyanion is provided by the element that is not oxygen. If there are two pos- sible groupings of the element with oxygen, the one with more oxygen atoms uses the suffi x -ate and the one with fewer oxygens uses the suffi x -ite. When there are four possible oxyanions, we add a prefi x per- to the -ate suffi x for the most oxygens and a prefi x hypo- to the -ite suffi x for the least oxygens. Chlorine is the classic example of an element that forms four oxyanions, whose names are provided in Table 2.7.
Generally, the bonds between the atoms within a polyatomic ion are covalent.
Generally, the bonds between the atoms within a polyatomic ion are covalent.
Table
❚
2.5Common cations Sodium ion Magnesium ion Iron(II) ion Iron(III) ion Silver ion Ammonium ion
Na+ Mg2+
Fe2+
Fe3+
Ag+ NH4+
Potassium ion Calcium ion Copper(I) ion Copper(II) ion Zinc ion Hydronium ion
K+ Ca2+
Cu+ Cu2+
Zn2+
H3O+
Table
❚
2.6Common anions Halides
Nitrate Phosphate Carbonate
F−, Cl−, Br−, I− NO3−
PO43−
CO32−
Sulfate Hydroxide Cyanide
Hydrogen carbonate
SO42−
OH− CN− HCO3−
Table
❚
2.7Oxyanions of chlorine ClO−
ClO2− ClO3−
ClO4−
Hypochlorite Chlorite Chlorate Perchlorate
Once we know how to name both of the ions, an ionic compound is named sim- ply by combining the two names. The cation is listed fi rst in the formula unit and in the name. Example Problem 2.6 provides some examples of the way to determine the name of an ionic compound.
E X A M P L E P RO B L E M 2 . 6
Determine the name of the following ionic compounds: (a) Fe2O3, (b) Na2O, (c) Ca(NO3)2
Strategy We must determine the names of the constituent ions fi rst. The anions will provide a hint about the cation charges if we need it.
Solution
(a) Fe2O3: As noted in Table 2.6, oxygen is always a 2− ion in these compounds, so there is a total charge of 6− on the three oxide ions in the formula unit. Therefore, the two iron ions must have a total charge of 6+, requiring 3+ from each iron. So the name is iron(III) oxide.
(b) Na2O: Sodium from Group 1 always has a 1+ charge and oxygen always 2−. Therefore, the name is sodium oxide. No Roman numeral is needed for sodium because it has only one common ionic charge.
(c) Ca(NO3)2: The calcium ion is in Group 2 and always carries a charge of 2+. NO3− is a common polyatomic anion called nitrate. The name is calcium nitrate.
Check Your Understanding Name each of the following ionic compounds:
(a) CuSO4, (b) Ag3PO4, (c) V2O5
Occasionally throughout this text, we will encounter new classes of chemical compounds that will need more rules to determine their names. We will introduce these nomenclature systems when necessary.
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