Page 16 - Radiochemistry and nuclear chemistry

Origin of Nuclear Science

dement - lead.
A major difficulty obvious to scientists at that time involved the fact that while it was
known from the Periodic Table (Appendix I) that there was space for only 11 dements
between lead and uranium, approximately 40 radioelements were known in the decay series
from uranium to lead. To add to the confusion was the fact that it was found that in many
cases it was not possible to separate some of the radioelements from each other by normal
chemical means. For example, the radioelement RaD was found to be chemically identical
to lead. In a similar manner, spectrographic investigations of the radioelement ionium
showed exactly the same spectral lines that had been found previously to be due to the
dement thorium.
In 1913 K. Fajans and Soddy independently provided the explanation for these seemingly
contradictory conditions. They stated that by the radioactive a-decay a new dement is
produceA two places to the left of the mother element in the periodic system and in B-decay
a new dement is produced one place to the fight of the mother element (Fig. 1.2). The
radioelements that fall in the same place in the periodic system are chemically identical.
Soddy proposed the name isotopes to account for different radioactive species which have
the same chemical identity.
Research by J. J. Thomson soon provided conclusive support for the existence of isotopes.
If a beam of positively charged gaseous ions is allowed to pass through electric or magnetic
fields, the ions follow hyperbolic paths which are dependent on the masses and charges of
the gaseous ions (see Fig. 2.1 and associated text). When these ion beams strike
photographic plates, a darkening results which is proportional to the number of ions which
hit the plate. By using this technique with neon gas, Thomson found that neon consists of
two types of atoms with different atomic masses. The mass numbers for these two isotopes
were 20 and 22. Moreover, from the degree of darkening of the photographic plate,
Thomson calculated that neon consisted to about 90% of atoms with mass number 20, and
10% of atoms with mass number 22.
Thus a chemical element may consist of several kinds of atoms with different masses but
with the same chemical properties. The 40 radioelements were, in truth, not 40 different
elements but were isotopes of the 11 different chemical dements from lead to uranium.
To specify a particular isotope of an element, the atomic number (i.e. order, number, or
place in the Periodic Table of elements) is written as a subscript to the left of the chemical
symbol and the mass number (i.e. the integer value nearest to the mass of the neutral atom,
measured in atomic weight units) as a superscript to the left. Thus the isotope of uranium
with mass number 238 is written as 2~U. Similarly, the isotope of protactinium with mass
number 234 is designated 2~4pa. For an alpha-particle we use either the Greek letter c~ or
~He. Similarly, the beta-particle is designated either by the Greek letter/3 or by the symbol
_0e.
In radioactive decay both mass number and atomic number are conserved. Thus in the
decay chain of 238T t the first two steps are written:
92"
23Stt92,., =') 23904Th + 4Hr (1.1)
234Th 234--
901,, "~ 9ira + _Oe (1.2)