be included as a superscript on the right side of the chemical symbol(s) when we write
the formula for the individual ion.
As discussed in detail in Chapter 5, an atom consists of a very small, very dense, pos-
itively charged nucleussurrounded by a diffuse distribution of negatively charged particles
called electrons.The number of positive charges in the nucleus defines the identity of the
element to which the atom corresponds. Electrically neutral atoms contain the same num-
ber of electrons outside the nucleus as positive charges (protons) within the nucleus. Ions
are formed when neutral atoms lose or gain electrons. An Naion is formed when a
sodium atom loses one electron, and a Clion is formed when a chlorine atom gains one
electron.
The compound NaCl consists of an extended array of Naand Clions (Figure 2-7).
Within the crystal (though not on the surface) each Naion is surrounded at equal dis-
tances by six Clions, and each Clion is similarly surrounded by six Naions. Any
compound, whether ionic or molecular, is electrically neutral; that is, it has no net charge.
In NaCl this means that the Naand Clions are present in a 11 ratio, and this is in-
dicated by the formula NaCl.
Because there are no “molecules” of ionic substances, we should not refer to “a
molecule of sodium chloride, NaCl,” for example. Instead, we refer to a formula unit
of NaCl, which consists of one Naion and one Clion. Likewise, one formula unit
of CaCl 2 consists of one Ca^2 ion and two Clions. As you will see in the next section,
we speak of the formula unit of all ionic compounds as the smallest, whole-number
ratios of ions that yield neutral representations. It is also acceptable to refer to a formula
unit of a molecular compound. One formula unit of propane, C 3 H 8 , is the same as one
molecule of C 3 H 8 ; it contains three C atoms and eight H atoms bonded together into
a group.
For the present, we shall tell you which substances are ionic and which are molecular
when it is important to know. Later you will learn to make the distinction yourself.
Polyatomic ionsare groups of atoms that bear an electric charge. Examples include
the ammonium ion, NH 4 ; the sulfate ion, SO 42 ; and the nitrate ion, NO 3 . Table 2-3
shows the formulas, ionic charges, and names of some common ions. When writing the
formula of a polyatomic compound, we show groups in parentheses when they appear
more than once. For example, (NH 4 ) 2 SO 4 represents a compound that has two NH 4
ions for each SO 42 ion.Figure 2-7 The arrangement of
ions in NaCl. (a) A crystal of sodium
chloride consists of an extended
array that contains equal numbers
of sodium ions (small spheres)and
chloride ions (large spheres). Within
the crystal, (b) each chloride ion is
surrounded by six sodium ions, and
(c) each sodium ion is surrounded by
six chloride ions.
54 CHAPTER 2: Chemical Formulas and Composition Stoichiometry
Na+Cl–Cl–Na+Na+(a)(b)(c)The general term “formula unit”
applies to molecular or ionic
compounds, whereas the more specific
term “molecule” applies only to
elements and compounds that exist
as discrete molecules.
In this text, we use the standard
convention of representing multiple
charges with the number before the
sign, e.g., Ca^2 , notCa^2 and SO 42 ,
notSO 4 ^2.