We use the information that the
carbonate ion has a 2charge to find
the charge on the iron ions. The total
charges must add up to zero.2-6 The Mole 57Solution
(a) The presence of the polyatomic grouping NH 4 in the formula suggests to us the presence
of the ammonium ion, NH 4 . There are two of these, each accounting for 1in charge. To
balance this, the single S must account for 2in charge, or S^2 , which we recognize as the
sulfide ion. Thus, the name of the compound is ammonium sulfide.
(b) The NO 3 grouping in the formula tells us that the nitrate ion, NO 3 , is present. Two of
these nitrate ions account for 2 1 2 in negative charge. To balance this, copper must
account for 2charge and be the copper(II) ion. The name of the compound is copper(II)
nitrate.
(c) The positive ion present is zinc ion, Zn^2 , and the negative ion is chloride, Cl. The name
of the compound is zinc chloride.
(d) Each CO 3 grouping in the formula must represent the carbonate ion, CO 32 . The pres-
ence of threesuch ions accounts for a total of 6in negative charge, so there must be a total
of 6present in positive charge to balance this. It takes twoiron ions to provide this 6, so
each ion must have a charge of 3and be Fe^3 , the iron(III) ion, or ferric ion. The name of
the compound is iron(III) carbonate.
You should now work Exercises 13 and 20.
A more extensive discussion on naming compounds appears in Chapter 4.ATOMIC WEIGHTS
As the chemists of the eighteenth and nineteenth centuries painstakingly sought infor-
mation about the compositions of compounds and tried to systematize their knowledge,
it became apparent that each element has a characteristic mass relative to every other
element. Although these early scientists did not have the experimental means to measure
the mass of each kind of atom, they succeeded in defining a relativescale of atomic masses.
An early observation was that carbon and hydrogen have relative atomic masses, also
traditionally called atomic weights (AW),of approximately 12 and 1, respectively.
Thousands of experiments on the compositions of compounds have resulted in the es-
tablishment of a scale of relative atomic weights based on the atomic mass unit (amu),
which is defined as exactly 112 of the mass of an atom of a particular kind of carbon atom, called
carbon-12.
On this scale, the atomic weight of hydrogen (H) is 1.00794 amu, that of sodium (Na)
is 22.989768 amu, and that of magnesium (Mg) is 24.3050 amu. This tells us that Na
atoms have nearly 23 times the mass of H atoms, and Mg atoms are about 24 times heav-
ier than H atoms.
When you need values of atomic weights, consult the periodic table or the alphabeti-
cal listing of elements, both found on facing pages inside the front cover.
THE MOLE
Even the smallest bit of matter that can be handled reliably contains an enormous num-
ber of atoms. So we must deal with large numbers of atoms in any real situation, and some
unit for conveniently describing a large number of atoms is desirable. The idea of using
a unit to describe a particular number (amount) of objects has been around for a long
time. You are already familiar with the dozen (12 items) and the gross (144 items).
2-6
2-5
The term “atomic weight” is widely
accepted because of its traditional use,
although it is properly a mass rather
than a weight. “Atomic mass” is often
used.