Water is a molecule composed of two hydrogen atoms and one oxygen
atom. It is not an element, so water per se does not have "isotopes".
Hydrogen and oxygen, however, both have more than one stable isotope,
and hydrogen has one moderately long-lived radioactive isotope
(tritium) that occurs at ultra-low abundances naturally (it's produced
by cosmic-ray interactions with the nuclei of other elements), and is
made in nuclear reactors for a variety of purposes. (For instance,
it's the "hydrogen" in a "hydrogen bomb".)
Oxygen has 3 stable isotopes with atomic masses 16, 17 and 18. All
isotopes of oxygen have nuclei containing 8 protons, and either 8, 9,
or 10 neutrons (corresponding to masses 16, 17, and 18.) O-16 makes
up about 99.76% of all oxygen on Earth, while O-17 and O-18 make up
only about 0.04% and 0.2%, respectively.
Hydrogen has stable isopes with atomic masses 1 and 2, and radioactive
tritium has mass 3. All have nuclei containing one proton and either
0, 1, or 2 neutrons. H-1 is most abundant, making up 99.985% of all
hydrogen on Earth, with H-2 making up 0.015%. Tritium (H-3) is only
present naturally at ultra trace levels (e.g., a few atoms in every
10^18 atoms of hydrogen). As noted above, man has generated tritium
that has escaped to the environment, and the current concentration of
tritium really depends on where you are making the measurement.
To a very good approximation, all the isotopes of a given element have
the same chemical behavior, so a water molecule doesn't really "care"
whether it contains an O-16, O-17, or O-18 atom. The same holds true
for the hydrogen atoms. This means that if we include tritium in our
considerations, any given water molecule has three isotopic
possibilities for the first hydrogen atom, three more for the second
hydrogen atom, and three possibilities for the oxygen atom. This
means that there are 3 x 3 x 3 = 27 isotopic "varieties" of water.
This may be what your book meant by there being "about 30 isotopes of
water".
(As an aside, it's not strictly true that all isotopes of a chemical
element have the same chemical behavior. There are small
mass-dependent differences in the chemical properties of different
isotopes, and these give rise to natural variations in the isotopic
composition of water in different "reservoirs". These effects are at
the few percent level for the different isotopes of oxygen, and can be
as much as 40% in the case of hydrogen. See
http://www.iapws.org/faq1/isotope.htm)
The water molecule itself has only one isomer, a bent structure with a
central oxygen and two hydrogens bonded to it and forming a ~104.5
degree angle. (see http://www.iapws.org/faq1/molecule.htm and
http://www.lsbu.ac.uk/water/) In liquid water, however, the individual
molecules can form relatively loosely bonded "clusters" with various
geometries that are constantly being rearranged. (Think of a cocktail
party in which indivuduals are constantly moving from one conversation
clusters to another. The size and makeup of each cluster varies as a
function of time.) The clusters are held together by the formation of
a type of chemical bond known as a "hydrogen bond", which is much
weaker than the bonds that hold the hydrogens to the oxygen in the
water molecule itself, but are nonetheless strong enough to cause
cluster formation. The nature (i.e., geometry, size, lifetime, etc.)
of these clusters is a topic of current research in water chemistry
(see http://www.lsbu.ac.uk/water/abstrct.html).
In solid water (i.e., ice and amorphous solid forms of water), the
water molecules can arrange themselves into a large number of possible
structures. The structure that's most energetically favorable, and
thus thermodynamically stable depends on the temperature and pressure
that the water is subjected to. There are at least 14 known
crystalline "polymorphs" of ice (polymorphs are different crystal
structures that have the same chemical composition), and probably 3
amorphous (i.e. glassy and non-crystalline) solid forms of water. See
http://www.lsbu.ac.uk/water/phase.html. |
