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Hydrogen is the simplest of atoms. How do more complex
atoms differ? To answer this question we must consider
more carefully the nature of the nucleus. We now know
that the nucleus itself comprises smaller particles,
protons and neutrons, whose mass is very similar. The
protons each have a positive electrical charge, whereas
the neutrons are uncharged. The proton charge is exactly
equal to but opposite in sign to that of the electron, (a
fact which surely hints at some underlying, more
fundamental unity in nature). An atom is a neutral entity
and must therefore have the same number of negatively
charged electrons as there are protons in the nucleus.
Different atoms are characterized by the numbers of
protons in their nuclei; the nucleus of the hydrogen atom
has one proton and hence traps one electron to yield the
neutral atom; helium has two protons and hence two
electrons; lithium three protons and three electrons.
Uranium, the heaviest naturally occurring element, has a
nucleus with 92 protons; heavier elements have been made
during the last 60 years in nuclear reactors and particle
accelerators.
Stable nuclei generally contain roughly equal numbers of
protons and neutrons, although in the heavier nuclei the
number of neutrons is generally greater. Conversely,
there are several known nuclei with the same number of
protons but with different numbers of neutrons. The
resulting atoms will have different masses but the same
number of electrons; their chemistry (which as we will
see depends on the number of electrons) is therefore
identical. They are known as ISOTOPES of the same
element.
Elementary electrostatics tells us that like charges
repel. We might therefore expect the protons in the
nucleus simply to fly apart. But the protons and neutrons
(nucleons as they are collectively known) are held
together by highly powerful attractive forces (known as
strong interactions) which, however, operate over very
small distances (just a few nucleon diameters). For this
reason there is a limit to the number of protons in
stable nuclei: as the charge builds up, the repulsive
forces between the protons which extend over much larger
distances begin to outweigh the attractive strong
interaction. Unstable nuclei are radioactive. They may
emit particles to achieve greater stability; for example,
many heavy atoms emit a-particles (helium nuclei
containing two protons and two neutrons) to reduce their
total charge. Dramatic examples of nuclear instability
are produced by the 'uranium-235' nucleus (which contains
92 protons and 143 neutrons - a total of 235
nucleons) which can split in two, the process of nuclear
fission, which is accompanied by a very large release of
energy.
To return, however, to the electrons which balance the
charge of the nucleus. These we have seen can occupy
'standing wave' states (or orbitals**) around the
nucleus; and these states have different energies. We
might expect that all the electrons would normally be
present in the lowest energy orbital. Once more, however,
the strangeness of the new physics confounds our
expectations. For we find that for particles like
electrons, it is impossible for exactly the same state to
be occupied more than once - the famous EXCLUSION
PRINCIPLE first enunciated by Pauli (which reflects
deeper aspects relating to the symmetries of wave
functions of systems containing many particles). An
additional twist is provided by the fact that electrons
(and many other particles) have a property known as SPIN
(which falls out naturally when relativity is included
into quantum mechanics and which is manifested by the
electron having a tiny magnetic moment). This can have
two values which we can call 'up' and 'down'. So we can
still obey the Pauli principle with two (but no more then
two) electrons in each orbital; for they are not exactly
in the same state: one has 'up' and one has 'down' spin.
We balance the charge of our nucleus by 'feeding' the
electrons in two by two into the orbitals - the
standing wave solutions of the Schrödinger equation
- starting with the lowest energy states. The
resulting atom is a highly complex object; it is a cloud
of electronic wave states interacting with each other but
bound to the nucleus.
The account given here is of course the briefest of
sketches of atomic structure; the interested reader will
find further information here. It is, however, worth
pausing at this point to reflect on the scale of the
intellectual achievement represented by our current
knowledge of the structure of atoms. As we recall, the
modern atomic theory started with John Dalton's
imaginative speculations stimulated by simple chemical
observation. Within 130 years the strange, sophisticated
but predictive model of atomic structure that we have
outlined above had been developed. This remarkable
success is a tribute to the creative power of the human
mind when engaged in the cooperative search for the truth
about the physical world that is the basis of science.
Finally, we will remember that the starting point for our
discussion of atomic structure was the search for the
ultimate constituents of matter. We have seen that atoms
are not indivisible; they have substructure; moreover,
that nuclei are made out of smaller particles -
protons and neutrons. The immense efforts in high energy
physics over the last fifty years have revealed large
numbers of elementary particles in addition to protons,
neutrons and electrons (although many of these have
lifetimes that are unimaginably short). In addition, it
is now clear that protons and neutrons have substructures
and appear to be made out of even smaller particles,
known as quarks. Whether quarks are indivisible or
themselves have substructures only time (and a great deal
of money) will tell.
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