p orbitals, this phase issue becomes important.
Let's have a look at another example of a
sigma bond. Let's take example hydrofluoric acid,
or HF. Note what we said before: The atom
configuration, or the electron configuration,
for hydrogen is 1s1. The electron configuration
for fluorine is, as you can see, 1s2, which
is a helium nucleus; 2s2; 2p5. Fluorine needs
to gain one electron; hydrogen can give one
electron. Or, as we've said, they can share
them covalently. And in this case, a hydrogen–fluorine
sigma molecular orbital can be formed. It's formed
from the interaction of a s atomic orbital
with, in this case, a p atomic orbital on
the fluorine. Note the difference in shape.
H has a single s orbital, shown in green.
The p orbital lobe, which must be in the direction
of the s orbital for the hydrogen, must also
be green, i.e. it must be in phase. When those
two orbitals overlap, it is possible to form
a sigma molecular bond. And this is shown here.
Note we have the s orbital of the hydrogen,
which was originally a sphere, now overlapping
with a p orbital of the fluorine. Also pay
attention to that lobe of the fluorine p orbital
which was out of phase, which is an orange
color. That lobe has now got a lot smaller.
This is still an example of a sigma orbital.
Bear in mind the orientation. Before, we were
talking about a p orbital which was in line,
which could facilitate overlap of the lobe
which was in phase with the s orbital. If,
on the other hand, we look at a p orbital,
which is at 90 degrees to that orbital we
were originally looking at, then you can see
that overlap of the in-phase lobe is no longer
possible. This cannot occur and indeed gives
a non-bonding interaction. And the best way
to explain this, the best way to find an analogy,
is to consider the idea of interference of
a radio signal. If you have two radio signals
which are coherent with each other, they result
in constructive interference, and this is,
in this case, desirable for interaction to
occur. If, on the other hand, your two signals
in the electromagnetic spectrum are out of
phase, you end up with destructive interference,
and this is undesirable.