The main purpose of this page is to provide the student with an opportunity to learn or review the rules that dictate how atoms may be joined together with covalent bonds to form legitimate molecules. Simultaneously, the student should learn to recognize drawings of molecules that could not exist, because one or more of the atoms shown exhibit impossible bonding patterns. Although it is assumed that the reader has fundamental understanding of bond formation between atoms, this page will also briefly review the differences between ionic and covalent bonds, as well as polar vs. nonpolar covalent bonds.
Beginning students frequently show they are confused about the possible ways that various atoms can be connected to each other within molecules. Often they will draw molecules that could not possibly exist, because the drawing contains an atom (or atoms) with unlikely or impossible combinations of bonds and lone pairs, and/or exceedingly high formal charges. Some examples of legal vs. impossible structures are shown below:
Self-test question #1
If you think you already know what's wrong with each of the "impossible" structures above, write down your answers now... but don't check until Self-Test Question #2.
Can you figure out what's wrong with each of the "impossible" structures shown in Figure 1? Write down your answers, then check them by clicking on the blue question mark below.
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Version 1.1.5, 3/21/97