General Chemistry Principles, Patterns, and Applications, 2011

formally belong to group 3, the most common ion formed by these elements is M 3+ , where M represents
the metal. Conversely, elements in groups 17, 16, and 15 often react to gain one, two, and three electrons,
respectively, to form ions such as Cl − , S 2− , and P 3− . Ions such as these, which contain only a single atom, are
called monatomic ions. You can predict the charges of most monatomic ions derived from the main group
elements by simply looking at the periodic table and counting how many columns an element lies from
the extreme left or right. For example, you can predict that barium (in group 2) will form Ba 2+ to have the
same number of electrons as its nearest noble gas, xenon, that oxygen (in group 16) will form O 2− to have
the same number of electrons as neon, and cesium (in group 1) will form Cs + to also have the same
number of electrons as xenon. Note that this method does not usually work for most of the transition
metals, as you will learn in Section 2.3 "Naming Ionic Compounds". Some common monatomic ions are
in Table 2.2 "Some Common Monatomic Ions and Their Names".
Note the Pattern
Elements in groups 1, 2, and 3 tend to form 1+, 2+, and 3+ ions, respectively; elements in groups 15, 16,
and 17 tend to form 3−, 2−, and 1− ions, respectively.
Table 2.2 Some Common Monatomic Ions and Their Names
Group 1 Group 2 Group 3 Group 13 Group 15 Group 16 Group 17
N 3−
Li +
Be 2+
nitride
O 2−
F −
lithium
beryllium
(azide)
oxide
fluoride
Na +
Mg 2+
Al 3+
P 3−
S 2−
Cl −
sodium
magnesium
aluminum
phosphide
sulfide
chloride
K +
Ca 2+
Sc 3+
Ga 3+
As 3−
Se 2−
Br −
potassium
calcium
scandium
gallium
arsenide
selenide
bromide
Rb +
Sr 2+
Y 3+
In 3+
Te 2−
I −
rubidium
strontium
yttrium
indium
telluride
iodide
Cs +
Ba 2+
La 3+
cesium
barium
lanthanum
E X A M P L E 3
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