7.1. Ionic Compounds http://www.ck12.org
Types of Formulas
Recall that a molecule includes two or more atoms that have been chemically combined.A chemical formula that
indicates how many of each type of atom are present in a single moleculeis referred to as amolecular formula.
For example, a molecule of ammonia contains one nitrogen atom and three hydrogen atoms, so it has the following
molecular formula:
Another type of chemical formula, theempirical formula,shows the elements in a compound in their lowest whole-
number ratio. Glucose is an important simple sugar that cells use as their primary source of energy. Its molecular
formula is C 6 H 12 O 6. Since each of the subscripts is divisible by 6, the empirical formula for glucose is CH 2 O. When
chemists analyze an unknown compound, often the first step is to determine its empirical formula, as we will see
in a later chapter. There are a great many compounds whose molecular and empirical formulas are the same. If the
molecular formula cannot be simplified into a smaller whole-number ratio, as in the case of H 2 O or P 2 O 5 , then the
empirical formula is also the molecular formula.
Ionic compounds are quite different from molecular compounds such as water. Water and other molecular com-
pounds exist as collections of individual molecules (Figure7.1).
FIGURE 7.1
A water molecule consists of one atom of oxygen bonded to two atoms of
hydrogen.Ionic compounds do not exist as discrete molecular units. Instead, an ionic compound consists of a large three-
dimensional array of alternating cations and anions. For example, sodium chloride (NaCl) is composed of Na+and
Cl−ions arranged into a structure like the one pictured below (Figure7.2).
Watch a video about The Structure of Ionic Solids: http://www.youtube.com/watch?v=TLPY9Z6z4Mg (1:35).
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