An Atomic-Level View of Elements and Compounds

A The basic units that compose chlorine gas are diatomic chlorine molecules. 

The highlighted elements exist primarily as diatomic molecules (yellow) or polyatomic molecules (red). 

A FIGURE 3.6 Molecular and Ionic Compounds (a) Propane is an example of a molecular componnd. The basic units that compose propane gas are propane (C3Hg) molecules, (b) Table salt (NaCl) is an ionic compound. Its formula unit is the simplest charge-nentral collection of ions one Na ion and one Cr ion. 

People occasionally refer to formula units as molecules, but this is not correct since ionic compounds do not contain distinct molecules. 

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