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## 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.

## 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 $MnO_4^-$