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Chapter 2

Q. 2.4

Naming an Ionic Compound from Its Formula

Name the following: a. Mg_3N_2, b. CrSO_4.

PROBLEM STRATEGY

Note that the compounds in a and b contain both metal and nonmetal atoms, so we expect them to be  ionic compounds. The first thing to do is write the formulas of the cations and anions in the compound, then name the ions. Look first for any monatomic cations and anions whose charges are predictable. Use the rules given in the text to name the ions. For those metals having more than one cation, you will need to deduce the charge on the metal ion (say the ion of Cr in CrSO_4) from the charge on the anion. You will need to memorize or have available the formulas and names of the polyatomic anions to be able to write them from the formula of the compound.

Step-by-Step

Verified Solution

a. Magnesium, a Group IIA metal, is expected to form only a 2+ ion (Mg^{2+}, the magnesium ion). Nitrogen (Group VA) is expected to form an anion of charge equal to the group number minus 8 (N^{3-}, the nitride ion). You can check that these ions would give the formula Mg_3N_2. The name of the compound is magnesium nitride (the name of the cation followed by the name of the anion).
b. Chromium is a transition element and, like most such elements, has more than one monatomic ion. You can find the charge on the Cr ion if you know the formula of the anion. From Table 2.6, you see that the SO_4 in CrSO_4 refers to the anion SO^{2-}_{4} (the sulfate ion). Therefore, the Cr cation must be Cr^{2+} to give electrical neutrality. The name of Cr^{2+} is chromium(II) ion, so the name of the compound is chromium(II) sulfate.

Table 2.6

Some Common Polyatomic Ions
Name Formula Name Formula
Mercury(I) or mercurous Hg_2^{2+} Nitrite NO_2^-
Ammonium NH_4^+ Nitrate NO_3^-
Cyanide CN^- Hydroxide OH^-
Carbonate CO_3^{2-} Peroxide O_2^{2-}
Hydrogen carbonate
(or bicarbonate)
HCO_3^- Phosphate PO_4^{3-}
Acetate C_2H_3O_2^- Monohydrogen phosphate HPO_4^{2-}
Oxalate C_2O_4^{2-} Dihydrogen phosphate H_2PO_4^-
Hypochlorite ClO^- Sulfite SO_3^{2-}
Chlorite ClO_2^- Sulfate SO_4^{2-}
Chlorate ClO_3^- Hydrogen sulfite
(or bisulfite)
HSO_3^-
Perchlorate ClO_4^- Hydrogen sulfate
(or bisulfate)
HSO_4^-
Chromate CrO_4^{2-} Thiosulfate S_2O_3^{2-}
Dichromate Cr_2O_7^{2-}
Permanganate MnO_4^-

ANSWER CHECK

Whenever you have to name a compound, you can check your answer to see if the name will lead you back to the correct formula.