7.1. Ionic Compounds http://www.ck12.org
FIGURE 7.1
A crystal of table salt, sodium chloride, is a large array of alternating
positive and negative ions. The purple spheres represent the Na+ions,
while the green spheres represent the Cl−ions.of sodium ions (Na+) and chloride ions (Cl−), so its name is sodium chloride. Ionic compounds like NaCl that
are composed of only two elements are referred to asbinary ionic compounds. Similarly, KOH is composed of
potassium ions (K+) and hydroxide ions (OH−), so its name is potassium hydroxide. Ionic compounds like KOH
that are composed of more than two elements are referred to asternary ionic compounds. To learn how to name
various ionic compounds, we simply need to learn the names of individual ions.
Monatomic Ions
Monatomic ionsform when a single atom gains or loses electrons. For the main group elements, cations are
generally formed by removing all of the valence electrons from the atom. Since the numbers of valence electrons
for the representative elements are constant within a particular group, all we need is the group number of a given
element to know its charge when it becomes a cation. Group 1 elements form ions with a 1+ charge, Group 2 metal
ions have a 2+ charge, and the ions of Group 13 elements tend to have a 3+ charge. Heavier p-block metals such as
tin and lead are special cases and will be discussed with the transition metal ions. The name of a monatomic cation
is the same as the name of the neutral element. For example, the sodium atom (Na) loses a single electron to form
the sodium ion (Na+), while Al^3 +is an aluminum ion.
Anions form when an atom gains 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 an
ns^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 end of the element’s name is dropped and
replaced with the –idesuffix. For example, when the chlorine atom (Cl) gains one electron, it becomes the chloride
ion (Cl−). This structure has the same electron configuration as the noble gas argon. Similarly, sulfur can gain two
electrons to become the sulfide ion (S^2 −), which also has a noble gas configuration.
Most main group elements, particularly those in groups 1, 2, 16, and 17, gain or lose enough electrons to form ions
that have the same electron configuration as that of the nearest noble gas.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−