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16. Multiple Bonding in N2 17. Summing Up 18. Atom-by-Atom Approach
17. Summing Up

In this chapter we've brought together just two atoms at a time, but we've learned a great deal about bonding even with that limitation. We've found that under many circumstances bonds don't form at all, and the two atoms simply repel one another. But we also found quite a few cases where bonding occurs.

One of the common themes is that almost all forms of bonding involve pairs of electrons, where the pairing can only happen if the spins of the electrons are different. Here is our list of bond types:

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Nonpolar Covalent: one electron from each atom, overlapping as allowed by the Pauli principle, with the electrons shared equally (e.g., H2 for electrons with opposite spins); both sigma and pi bonding is possible.

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Polar Covalent: one electron from each atom, overlapping as allowed by the Pauli principle, with the electrons shared unequally and polarized toward one of the atoms (e.g., HF for electrons with opposite spins); both sigma and pi bonding is possible.

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Ionic: one electron from each atom, overlapping as allowed by the Pauli principle, with the pair mostly residing on an electron hungry atom (e.g., LiH)

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Recoupled pair: a conditional form of bonding where an atom (or molecule) with a singly occupied orbital is able to break apart an existing pair of electrons in order to form a bond with one of them (e.g., BeH)

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Dative: a shared electron pair provided by just one of the atoms (e.g., HeH+)

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One-electron: a single, shared electron (e.g., H2+)

Note that the descriptions for both forms of covalent bonding now mention that sigma and pi bonding are possible.

Of the list of bond types, only one-electron bonds don't involve at least a pair electrons. Three electrons are involved in a recocupled pair bond, but two of them are always paired together (though the pairing itself changes!).

Another recurring them is that some bonds are polarized while others are not. We've only seen non-polar bonding in cases where the two atoms are the same, including H2 and N2. We've also observed that other pairs of electrons move in response to bond formation, such as when the 2s2 pairs on both N atoms move away from the bond region when N2 forms.

In the next chapter, we will begin to explore what happens when more than two atoms come together. We will start with an atom (like O) and keep adding other atoms (such as H) until nothing bigger can form.

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