7.1. Ionic Compounds http://www.ck12.org
Anions
Monatomic anions are negative ions that are formed when a nonmetal atom gains one or more electrons. Nonmetallic
atoms typically gain enough electrons to obtain the same electron configuration as the nearest noble gas. All the
elements in Group 17 have seven valence electrons, which are arranged into a outer configuration ofns^2 np^5. To
achieve a noble gas configuration (ns^2 np^6 ), each of these elements needs to gain just one electron, resulting in an
anion with a 1−charge. Similarly, Group 16 elements can obtain anns^2 np^6 valence configuration by forming ions
with a 2−charge, and the Group 15 nonmetals will form ions with a 3−charge. Naming anions is slightly different
than naming cations. The ending of the element’s name is dropped and replaced with the –idesuffix. For example,
F−is the fluoride ion, while O^2 −is the oxide ion. The table below (Table7.1) shows the names and charges for
common monatomic ions of the representative elements.
TABLE7.1: Common Monatomic Ions
1+ 2+ 3+ 3- 2- 1-
lithium, Li+ beryllium, Be^2 + aluminum, Al^3 + nitride, N^3 − oxide, O^2 − fluoride, F−
sodium, Na+ magnesium,
Mg^2 +gallium, Ga^3 + phosphide, P^3 − sulfide, S^2 − chloride, Cl−potassium, K+ calcium, Ca^2 + arsenide, As^3 − selenide, Se^2 − bromide, Br−
rubidium, Rb+ strontium, Sr^2 + telluride, Te^2 − iodide, I−
cesium, Cs+ barium, Ba^2 +Transition Metal Ions
Most transition metals differ from the metals of Groups 1, 2, and 13 in that they are capable of forming more than one
type of stable cation. For example, iron sometimes loses two electrons to form the Fe^2 +ion, but it is also common
for iron to lose three electrons to form the Fe^3 +ion. Although they are members of thepblock and not thedblock,
tin and lead also form more than one type of ion. Because the charges of these ions cannot be unambiguously
determined by looking at the periodic table, they must have names that also indicate their charge. The Stock system
denotes the charge of these ions by using a Roman numeral in parentheses after the name of the ion. For example,
the previously mentioned iron ions are named the iron(II) ion and the iron(III) ion. When said out loud, "iron(II) ion"
should be read, “iron two ion.” Listed below (Table7.2) are the names and formulas of some of the more common
transition metal ions.
TABLE7.2: Common Transition Metal Ions
1+ 2+ 3+ 4+
copper(I), Cu+ cadmium, Cd^2 + chromium(III), Cr^3 + lead(IV), Pb^4 +
gold(I), Au+ chromium(II), Cr^2 + cobalt(III), Co^3 + tin(IV), Sn^4 +
mercury(I), Hg 22 + cobalt(II), Co^2 + gold(III), Au^3 +
silver, Ag+ copper(II), Cu^2 + iron(III), Fe^3 +
iron(II), Fe^2 +
lead(II), Pb^2 +
manganese(II), Mn^2 +
mercury(II), Hg^2 +
nickel(II), Ni^2 +
platinum(II), Pt^2 +
tin(II), Sn^2 +
zinc, Zn^2 +