Question 8.2: Predict the ion generally formed by (a) Sr, (b) S, (c) Al.
Predict the ion generally formed by (a) Sr, (b) S, (c) Al.
Analyze We must decide how many electrons are most likely to be gained or lost by atoms of Sr, S, and Al.
Plan In each case, we can use the element’s position in the periodic table to predict whether the element forms a cation or an anion. We can then use its electron configuration to determine the most likely ion formed.
Solve
(a) Strontium is a metal in Group 2 and therefore forms a cation. Its electron configuration is [Kr] 5s^{2}, and so we expect that the two valence electrons will be lost to give an Sr^{2+} ion.
(b) Sulfur is a nonmetal in Group 16 and will thus tend to be found as an anion. Its electron configuration ([Ne] 3s^{2} 3p^{4}) is two electrons short of a noble-gas configuration. Thus, we expect that sulfur will form S^{2-} ions.
(c) Aluminum is a metal in Group 13. We therefore expect it to form Al^{3+} ions.
Check The ionic charges we predict here are confirmed in Tables 2.4 and 2.5
| TABLE 2.4 Common Cations^{a} | ||||
| Charge | Formula | Name | Formula | Name |
| 1+ | H^{+} | hydrogen ion | NH^{+}_{4} | ammonium ion |
| Li^{+} | lithium ion | Cu^{+} | copper(I) or cuprous ion | |
| Na^{+} | sodium ion | |||
| K^{+} | potassium ion | |||
| Cs^{+} | cesium ion | |||
| Ag^{+} | silver ion | |||
| 2+ | Mg^{2+} | magnesium ion | Co^{2+} | cobalt(II) or cobaltous ion |
| Ca^{2+} | calcium ion | Cu^{2+} | copper(II) or cupric ion | |
| Sr^{2+} | strontium ion | Fe^{2+} | iron(II) or ferrous ion | |
| Ba^{2+} | barium ion | Mn^{2+} | manganese(II) or manganous ion | |
| Zn^{2+} | zinc ion | Hg^{2+}_{2} | mercury(I) or mercurous ion | |
| Cd^{2+} | cadmium ion | Hg^{2+} | mercury(II) or mercuric ion | |
| Ni^{2+} | nickel(II) or nickelous ion | |||
| Pb^{2+} | lead(II) or plumbous ion | |||
| Sn^{2+} | tin(II) or stannous ion | |||
| 3+ | Al^{3+} | aluminum ion | Cr^{3+} | chromium(III) or chromic ion |
| Fe^{3+} | iron(III) or ferric ion | |||
| ^{a}The ions we use most often in this course are in boldface. Learn them first. | ||||
| TABLE 2.5 Common Anions^{a} | ||||
| Charge | Formula | Name | Formula | Name |
| 1- | H^{-} | hydride ion | CH_{3}COO^{-} (or C_{2}H_{3}O^{-}_{2}) | acetate ion |
| F^{-} | fluoride ion | ClO^{-}_{3} | chlorate ion | |
| Cl^{-} | chloride ion | ClO^{-}_{4} | perchlorate ion | |
| Br^{-} | bromide ion | NO^{-}_{3} | nitrate ion | |
| I^{-} | iodide ion | MnO^{-}_{4} | permanganate ion | |
| CN^{-} | cyanide ion | |||
| OH^{-} | hydroxide ion | |||
| 2- | O^{2-} | oxide ion | CO^{2-}_{3} | carbonate ion |
| O^{2-}_{2} | peroxide ion | CrO^{2-}_{4} | chromate ion | |
| S^{2-} | sulfide ion | Cr_{2}O^{2-}_{7} | dichromate ion | |
| SO^{2-}_{4} | sulfate ion | |||
| 3- | N^{3-} | nitride ion | PO^{3-}_{4} | phosphate ion |
| ^{a}The ions we use most often are in boldface. Learn them first. | ||||
