Video Transcript
The following figure shows five
silicon atoms that are part of a silicon lattice. Only electrons in the outer shells
of the atoms are shown. The two atoms shown within the
pink-dashed region are a bonded lattice pair. Which, if any, of the electrons of
either of the atoms of the pair is a free electron? Is it (A) electron two only, (B)
electrons one and five only, (C) all of the labeled electrons, or (D) none of the
labeled electrons?
In our diagram, we see that six of
the electrons within this pink-dashed line are indicated. Here we see electron one. Here is the electron labeled
electron two. Here is number three, four, five,
and six. We want to figure out which of
these, if any, is a free electron.
Recall that a free electron is one
that energetically is not bound to any particular atom. In that sense, a free electron
exists outside of the electron orbitals of an atom.
To identify free electrons in our
diagram, we’ll look for those electrons shown by blue dots that are not located on
the black circles that indicate electron energy levels. Looking first at electron one, we
see that this electron is located at the intersection of the orbitals of two silicon
atoms. This electron is shared between
these atoms. And it is a bound electron rather
than a free one.
Since electron one is not a free
electron, we can eliminate answer option (B), which suggests that electron one is
free. We can also eliminate answer option
(C), which says that all of the labeled electrons are free.
Next, we look at electron two. This blue dot, we notice, does not
lie along any of the black circles. It exists outside of those
circles. And indeed we can even see an
opening, called a vacancy, where this electron may have existed. Currently though, electron two does
exist outside the orbit of any particular atom. Therefore, it is a free
electron. Looking back at our answer options,
this means that option (D), which says that none of the labeled electrons are free,
is incorrect.
Before settling on our final answer
though, let’s look at the remaining electrons. Electron three exists on a black
circle, as does electron four, indicating that they are bound electrons. Electron five exists at the overlap
between two black circles. This electron is bound, and it is
shared. The same is true for electron six,
which is shared between our central silicon atom and the atom directly below it.
Answer choice (A) then is our final
answer. Of all of the six electrons
indicated, it’s only electron two that is a free electron.
