Shells and Orbitals
Electrons supposedly move around
the nucleus of atoms. For a long time it remained a mystery how they would
do such a thing1. At the beginning
of the 20th Century a series of clever suppositions resulted in the
electron shell theory, which is merely an extension of everyday quantum
mechanics2. According to this
theory, electrons move randomly around the nucleus, concentrated within
regions called 'orbitals'. Orbitals are defined as the volume where an
electron is most likely to be, and which bear more resemblance to a cloud
or a propeller, than to the orbit of a planet around a star. These
orbitals have defined shapes and the electrons moving within them obey
Orbitals are classified according to their shape, with
a letter code in order of increasing shape complexity: s, p, d, f, g, h,
i... The original designation of those orbitals was done according to
their spectral appearance, namely: sharp,
fundamental, hence the letter code with the
remaining letters being ordered alphabetically. To cause even greater
confusion, orbitals always come in groups. There is one s-orbital, three
p-orbitals, five d-orbitals, seven f-orbitals, and so on. All of these
orbitals are arranged forming shells which also have number and
letter-codes starting from the closest to the nucleus:
|Set of orbitals letter
It should be stressed that the
existence of these orbitals is independent from the presence of electrons.
Or, in other words, the orbitals are always present whether or not there
are any electrons about to fill them.
How to Fill Shells and Orbitals with
The next thing to consider is how
to fill these shells and orbitals with electrons. Doing this for the
neutrally charged atoms will result in the arrangement of the periodic
table of the elements. This is also the reason why the elements are
arranged in that particular way on the table. The period number is also
the shell number, and the group number is associated with the filling of
the orbitals (eg, the transition metals are sometimes called d-block
elements, because their d-orbitals are being filled).
As mentioned, it is difficult to visualize the
behavior of electrons. One consequence of this theory is that there are
only certain defined orbitals in which the electrons can move. Another one
is that only two electrons (each having a quality called 'spin', but which
has nothing to do with real-life spinning) can occupy one orbital at a
time. This theory is known as the Pauli Exclusion Principle, named after
the Austrian scientist Wolfgang Pauli (1900-1958) who invented it in 1925
and received a Nobel prize in 1945.
Scientists have calculated the shapes and positions of
the orbitals, and the energy electrons would have if they were moving in
the respective orbitals. They have concluded that electrons will tend to
move in a shell as close as possible to the nucleus and in an orbital with
a shape as simple as possible. However, just to make things a little bit
more complicated, in certain cases the shape of the orbital is so
complicated that the electron would rather take a simpler orbital in a
more distant shell than having to move within a complicated shape.
A simple orbital filling order scheme has been found to
which there are only minor exceptions. Electrons will fill the orbitals in
the following order:
1s(2), 2s(2), 2p(6), 3s(2), 3p(6), 4s(2), 3d(10), 4p(6), 5s(2), 4d(10)3...
The number in parenthesis corresponds to the maximum
number of electrons in the set of orbitals (always two per orbital, but
there are three p-orbitals, five d-orbitals, and so on). By way of
example, Iron (uncharged) has 26 electrons, so its electronic
configuration would be:
1s(2) 2s(2) 2p(6) 3s(2) 3p(6) 4s(2) 3d(6)
Adding the numbers in the parentheses will give 26. As
for notation, the complete s- and p-orbitals can be abbreviated using the
symbol of the corresponding noble gas4.
In our example, Iron would be: [Ar] 4s(2) 3d(6)
Applications of the Principle
The existence of defined orbitals and shells can be used
in technology in numerous ways.
To analyze the atomic composition of anything (from fruit cakes to
atmospheres of planets to sun-spots)
To generate colorful light in neon, xenon or mercury-lamps (see also
emission lines) and lasers
To calculate the shape of molecules like enzymes and Aspirin
To predict properties of metals, like their usage in photovoltaic devices
To calculate properties of doped and undoped semi-conductors (which are
used to build microchips)
1. Even more obscure is the
question why they would do something like that (see also: wave-particle
2. Which is a perversely complicated piece of
science, which 'only three or four people on this planet understand',
according to RP Feynman.
3. In some textbooks the number in the parentheses
can be found superscripted (like 4d7), but that is a minor detail.
4. The element on the periodic table at the far
right side, and one line above.