Video Transcript
If the first shell of an atom is
filled, but no other shells contain any electrons, how many electrons are there in
the atom?
In this question, we’ve been asked
to work out how many electrons an atom contains if its first shell is filled. To answer this question, we need to
recall that atoms consist of a nucleus and some number of electrons, which are found
outside the nucleus.
We often see atoms represented by
diagrams like this, where the nucleus is in the center and the electrons are placed
on rings around the nucleus. These rings represent a very
important concept: electron shells. These shells give us information
about how much energy each electron has. And it turns out that there are
some strict rules about how many electrons can be in each shell. The first electron shell can
contain a maximum of two electrons. If there are any more than two
electrons in an atom, the electrons will have to start filling the second shell and
then the third shell, and so on.
In this question, we are told that
the first shell of an atom has been filled by electrons but that there are no other
electrons in any of the other shells. In other words, the atom contains
just enough electrons to fill the first shell. It takes two electrons to fill the
first shell, so the answer to this question must be two. If there were fewer than two
electrons, the first shell would not be full. If there were more than two
electrons, there would be at least one electron in one of the other shells. So, the only possible answer is
that there are two electrons in the atom.
