Yes, there are some definite rules about this, but first we should
give a warning. These pictures are just a way of suggesting some
information about the atoms. The atoms themselves are not put together
in anything like the way the picture suggests, or even any way that can
be pictured at all. We have some other answers discussing a little bit
of these quantum mysteries.
Of course, the total number of electrons in an atom will be the
same as the number of protons in the nucleus, so that the atom is
The rings for the electrons can be used to suggest how many
electrons there are with different amounts of energy. The inner rings
represent electrons with low energy, tightly stuck in the atom. I don't
know what type of pictures your book favors, but they may also use
separate rings to represent electrons which are in different types of
states around the nucleus, even when those states have almost the same
The innermost ring can have one or two electrons in it. If that
fills up, the next batch can have up to eight electrons in it.
Depending on the taste of the illustrator, those may be shown as one
ring of two and one of six, or as one ring of eight, since six of those
states have a different 'shape' than the other two.
I'm not sure how far up you need to go, but the ring picture starts to get messy if you go much farther.
There's reason for the electrons being shown in pairs. There can be
at most one electron in any state. That's why they can't all pile into
the low-energy states. However, each state is determined not just by
how the electron is distributed in space but also by an internal
property called 'spin'. You can put two electrons in the same spatial
state so long as they have opposite spin. That's where the pairs come
(republished on 07/21/06)