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I was reading Peter Atkins' Elements of Physical Chemistry where I came across this line:

Bonds do not form because electrons tend to pair; bonds are allowed to form by the electrons pairing their spins.

What does this line mean? What does Atkins want to say through this line & why did he italize the words I bolded above?

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I dunno if the author meant exactly this, but it is a well-known fact that pairing of electrons actually consumes energy.

For example, free nitrogen atom in ground state has three unpaired electrons. To form triple bond in dinitrogen molecule, three pairs of electrons must be, well, paired. The process of pairing does consume energy. However, the energy freed in the bond formation is so big, that it overwhelms pairing energy by huge margin. On the other hand, in oxygen molecule two highest-energy electrons has two orbitals to occupy, and they stay unpaired, so dioxygen is paramagnetic.

So, by no means it is the fact of being unpaired that drives electrons to form bonds. In fact, it prevents formation of weak bonds. However, when two atoms come closer to each other and have two half-occupied orbitals that can form a bond, the fact that electrons may be paired allows this two orbitals to combine into occupied bonding orbital, forming a bond.

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  • $\begingroup$ How does the spin enter the picture? $\endgroup$
    – Todd Minehardt
    Jun 25, 2015 at 17:25
  • $\begingroup$ @ToddMinehardt going with huge simplification, the pauli principle states that no two electrons can have exactly same state. Electrons on same orbital must have exactly same wavefunction, however as they have spin, they still can be in states, characterised by same wavefunction, but with different (opposite) spin orientation ('up' and 'down'). Thus, they may be 'paired' $\endgroup$
    – permeakra
    Jun 25, 2015 at 17:44
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    $\begingroup$ @ToddMinehardt Orbital != Spin-orbital ; Spin-wavefuction != wavefunction. Two electrons on same orbital have same '3d-wavefunction', or, if you are annoyingly nitpicking, same 3d component of the wavefunction but different spin component. I've no idea what Atkins said and don't care about it at all, I wrote how the things are. $\endgroup$
    – permeakra
    Jun 25, 2015 at 18:09
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    $\begingroup$ @permeakra This is coming across as somewhat rude. Please be mindful of how your comments may be perceived. Thanks! $\endgroup$
    – jonsca
    Jun 25, 2015 at 23:00
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    $\begingroup$ Some actual numbers for the cost of electron pairing can be found in this answer of mine. The magnitude of the pairing energy is usually around 2 - 3 eV. $\endgroup$
    – Philipp
    Jun 27, 2015 at 9:01

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