I think you're asking about different isotopes, such as carbon 12 and
carbon 13. For a given number of protons (say 6), different numbers of
neutrons (say 6 or 7) can bind with the protons to form stable nuclei.
So these are really different nuclei. Why then do we call the atoms
that they make when they bind electrons atoms of the same element (say
carbon again)? The total electrical charge on the nucleus is determined
by the number of protons. The number of electrons bound in a neutral
atom then just matches the proton number. Most of the chemical behavior
is determined by these bound electrons. Only some relatively subtle
chemical effects depend on the mass of the nuclei. So we lump together
these atoms with similar chemistry but slightly different masses under
the name of a single element.
Although the effects of isotope mass on chemistry are usually
subtle, they sometimes are quite prominent. That's particularly true
for hydrogen and its isotope deuterium, which differ by a factor of two
in mass. That's enough to effect chemical reaction rates and equilibira
a lot. I believe that if you tried switching to drinking heavy water
(with deuterium) it would be enough different to be fatal. On the other
hand, some electrical engineers here have found that using deuterium
rather than hydrogen in processing silicon chips can dramatically
extend the chips' lifetime. The different rates of biochemical
reactions for different isotopes leave identifiable isotope
concentration changes in biological residues, which can be handy in
analyzing deposits.
Mike W.
On the more general question of why there are different kinds of
elements: Protons and neutrons stick together with the strong nuclear
force. Many different combinations of protons and neutrons can be bound
together with this very strong force, but some combinations are more
stable than others. If you put too many together, then a nucleus can
have more energy being put together than the energy of two smaller
nuclei plus a little kinetic energy. The nuclei of atoms like these
spontaneously fall apart -- usually into an alpha particle (two protons
and two neutrons bound together very stably) and whatever's left. This
process may repeat. Known stable nuclei have to be stable with respect
to alpha decay -- the stablest have this decay energetically
prohibited.
Another kind of nuclear decay is called beta decay, and its cousin,
inverse beta decay or inner-shell electron capture. If there are many
more neutrons than protons for example, it can be energetically
favorable for a neutron to decay into a proton by emitting an electron
and a neutrino, and thus fall into a lower energy level (protons and
neutrons fill up separate energy levels inside the nucleus. If the
neutron ones are filled up to a higher level than the protons, energy
can be gotten by this decay process). Protons can turn into neutrons by
grabbing an electron and emitting a neutrino. These only happen if the
energy works out to allow it.
It turns out that these two processes, alpha decay and beta decay,
allow only a subset of the possible combinations of protons and
neutrons to form stable nuclei. The stablest nuclei have approximately
as many protons as neutrons, and elements with more than about 100
protons are all unstable. Different isotopes of common elements can
have different lifetimes and decay to other elements.
Tom
(published on 10/22/2007)
