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PHYSICAL AND MOLECULAR INTERACTIONS 47

His observations also led him to coin two terms that still persist in present-day
genetics: dominance for a trait that shows up in an offspring, and recessiveness for a
trait masked by a dominant gene.

How do we make liquid nitrogen?

London dispersion forces

Liquid nitrogen is widely employed when freezing sperm or eggs
when preparing for the in vitro fertilization (IVF). It is also essential A ‘homonuclear’ mole-
cule comprises atoms
for maintaining the cold temperature of the superconducting magnet
at the heart of the NMR spectrometers used for structure elucidation from only one element;
homo is Greek for
or magnetic resonance imaging (MRI). ‘same’. Most molecules
◦
Nitrogen condenses to form a liquid at −196 C (77 K), which
are ‘heteronuclear’ and
is so much lower than the temperature of 373.15 K at which water comprise atoms from
condenses that we suspect a different physicochemical process is in several elements; het-
◦
evidence. Below −196 C, molecules of nitrogen interact, causing ero is Greek for ‘other’
condensation. That there is any interaction at all should surprise us, or ‘different’.
because the dipoles above were a feature of heteronuclear bonds,
but the di-nitrogen molecule (N≡N) is homonuclear, meaning both
atoms are the same.
We need to invoke a new type of interaction. The triple bond
Dipoles are usually a
between the two nitrogen atoms in the di-nitrogen molecule incor- feature of heteronu-
porates a huge amount of electron density. These electrons are clear bonds, although a
never still, but move continually; so, at any instant in time, one fuller treatment needs
end of a molecule might be slightly more negative than the other. to consider the elec-
A fraction of a second later and the imbalance departs. But a tiny tronic environment
dipole forms during the instant while the charges are unbalanced: of atoms and groups
we call this an induced dipole; see Figure 2.6. beyond the bond of
interest.
The electron density changes continually, so induced dipoles
never last more than about 10 −11 s. Nevertheless, they last suf-
ficiently long for an interaction to form with the induced dipole of another nitrogen
molecule nearby. We call this new interaction the London dispersion force after Fritz
London, who first postulated their existence in 1930.
London dispersion forces form between all molecules, whether polar or non-polar.
In a large atom or molecule, the separation between the nucleus and valence elec-
trons is quite large; conversely, the nucleus–electron separation in a lighter atom or
molecule is smaller, implying that the electrons are more tightly held. The tighter
binding precludes the ready formation of an induced dipole. For this reason, larger
(and therefore heavier) atoms and molecules generally exhibit stronger dispersion
forces than those that are smaller and lighter.