Active Learning In Chemistry Education - Potomac School

27
h. Al
13
4
+ He -----> +
2
30
P
15
i.
239
U
92
239
-----> Np +
93
The radioactive elements with atomic numbers 84 through 92 (up to and including uranium) have
some naturally-occurring radioactive isotopes. The elements beyond uranium (with atomic numbers
greater than 92) do not have any naturally occurring isotopes. These elements beyond uranium are
known as the transuranium elements. They are all synthetic elements since all of their isotopes are manmade.
Most of the radioactive elements (with atomic numbers 84 and above) are too unstable to be
assigned an atomic mass (atomic weight). If you look at a periodic table, you will notice that the atomic
masses of these elements are given in parentheses. (Check this out on a periodic table now.) The
number in the parentheses represents the atomic mass of the single most stable isotope. You will recall
that atomic mass is defined as the average mass of the various naturally occurring isotopes of an element
in the proportions in which they occur in nature. The radioactive isotopes of elements with atomic
numbers 84 and above are constantly decomposing. These isotopes have different half-lives, which
means that they are decomposing at different rates. Use this information to explain why these elements
cannot have an atomic mass as defined above:
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Most elements with atomic numbers smaller than 84 are stable because NONE of their naturallyoccurring
isotopes are radioactive. There are some exceptions to this rule. For example, K-40 and Ca-46
are radioactive. Most of the elements below atomic number 84 are stable enough to be assigned an
atomic mass. (Elements #43 and #61, Technetium and Promethium, are exceptions.) Man-made
radioactive isotopes have been synthesized for many of these elements, but synthetic isotopes are not
included in the calculation of atomic masses since they are not found in nature.
Section 28.3 Early Studies of Radioactivity
In1896, a French scientist by the name of Henri Becquerel accidentally discovered natural
radioactivity while conducting experiments with a uranium compound called potassium uranyl. In one of
his experiments, Becquerel wrapped a photographic plate in black, lightproof paper and placed some of
the uranium compound on top of the covered plate. He then placed this arrangement in the sunlight.
Although the sunlight could not pass through the lightproof paper, the plate became exposed in the area
of the uranium compound, as indicated by a dark area on the photograph. Becquerel thought that
perhaps energy from the sun had been changed into some more penetrating form which was able to pass
through the paper. He then attempted to repeat the experiment, but cloudy weather prevented him from
doing so at that time. He decided to store his second set-up in a closed drawer. Later, on a sunny day,
Becquerel repeated the experiment using a fresh photographic plate instead of the one he had stored in
the closed drawer. He then developed both of the photographic plates. Since the stored plate had not
been exposed to sunlight, Becquerel expected the developed photograph to be blank or almost blank.
Instead, he found that it had a dark area like that of the fresh plate which had been exposed to sunlight.
Becquerel reasoned that the uranium compound must have emitted some type of energy on its own
without the stimulation of sunlight. This ability of a nucleus to emit energy spontaneously (without
external stimulation) is called natural radioactivity. Uranium ore exhibits natural radioactivity with the
greatest amount of energy coming from its most abundant naturally-occurring isotope, U-238.
28-9 ©1997, A.J. Girondi