Notes on Ionic Compounds
I.
Rules for the formation of ions for the A-group
elements
NOTE: The number of valence electrons for an A-group
element is equal to the group number.
a) Metals lose electrons until the valence shell
electronic configuration is s2p6 (s2 for Li+ and Be2+). Thus
the charge on an ion formed from an A-group metal is equal
to its group number.
b) Nonmetals gain electrons until the valence shell
electronic configuration is s2p6 . Thus the charge on an ion
formed from an A-group nonmetal is equal to
(group number) – 8
(H+ is an exception that we will study later.)
II.
Ions formed from transition metals (B-group elements)
Some transition metals form more than one type of ion.
For these you must learn the different ions and their
names; for example, copper (Cu+ and Cu2+), and iron (Fe2+ and
Fe3+).
III. Polyatomic ions
Polyatomic ions are ions containing two or more atoms,
almost always nonmetals. The polyatomic ions we will study
are listed in Table 5.7 on page 180 of the text.
IV.
Writing formulas for ionic compounds
The formula for an ionic compound gives the number of
each ion present in the compound. Such a formula is a
“simplest” formula; the numbers are integers, and give the
simplest whole-number ratios of the ions present.
ion.
a) Write the positive ion first, then the negative
+
(Except in the case of the ammonium ion, NH4, the positive
ion is always a metal ion.)
b) The formula may include more than one of the
positive ion and/or more than one of the negative ion.
a particular ion, indicate the number present with a
subscript to the right of the symbol for the ion.
For
**Choose the subscripts so that
total positive charge = total negative charge.
c) If a subscript is needed for a polyatomic ion,
enclose the polyatomic ion in parentheses, and put the
subscript outside the parentheses. (For counting the
number of atoms of each kind, remember that a subscript on
a polyatomic ion multiplies each atom in the ion.) NOTE:
If only one of a particular polyatomic ion is present in
the formula, then the parentheses around that ion are
omitted.
V.
Naming ionic compounds (Nomenclature)
a) Write the name of the positive ion first, then the
name of the negative ion.
+
b) Except for NH4, the positive ion is formed from a
metal. The name of a metal ion is the same as the name of
the metal itself, unless the metal forms more than one type
of ion; then the name depends on the charge on the ion.
For example,
Cu+
cuprous
Cu2+
cupric
Fe2+
ferrous
Fe3+
ferric
c) If the negative ion is formed from a single atom,
the name of the ion is obtained from the name of the atom
by changing the ending of the atom name to "ide".
d) For polyatomic ions, the names, formulas, and
charges must be learned. See Table 5.7 on page 180.
Examples:
MgI2 is magnesium iodide.
Na3PO4 is sodium phosphate.
(NH4)2CO3 is ammonium carbonate
CuCl is cuprous chloride
CuCl2 is cupric chloride
Fe(HSO4)2 is ferrous hydrogen sulfate [also called ferrous
bisulfate]