antly changing
into other elements, thereby releasing very large amounts of energy, and
that many different forms of the same element (lead was the first
studied) were possible. We now think of an element as a form of matter
in which all atoms have the same nuclear charge.

The human mind has always sought order and simplification of the
external world; in chemistry the fruitful classifications were
Dobereiner's Triads (1829), Newland's law of octaves (1865), and
Mendeleev's periodic law (1869). The chart expressing this periodic law
seemed to indicate the maximum extent of the elements and gave good
hints "where to look for" and "the probable properties of" the remaining
ones (see Fig. 2).

By 1925, all but four of the slots in the 92-place file had been filled.
The vacancies were at 43, 61, 85, and 87.

[Illustration: Fig. 2. Periodic chart of the elements (1963)]

Workers using traditional analytical techniques continued to search for
these elements, but their efforts were foredoomed to failure. None of
the nuclei of the isotopes of elements 43, 61, 85, and 87 are stable;
hence weighable quantities of them do not exist in nature, and new
techniques had to be developed before we could really say we had
"discovered" them.

In 1919, Rutherford accomplished scientifically what medieval alchemists
had failed to do with "magic" experiments and other less sophisticated
techniques. It wasn't gold (the goal of the alchemists) he found but
something more valuable with even greater potential for good and evil: a
method of transmuting one element into another. By bombarding nitrogen
nuclei with alpha particles from radium, he found that nitrogen was
changed into oxygen.

The process for radioactive transmutation is somewhat like a common
chemical reaction. An alpha particle, which has the same charge (+2) and
atomic mass (4) as a helium nucleus, penetrates the repulsive forces of
the nitrogen nucleus and deposits one proton and one neutron; this
changes the nitrogen atom into an oxygen atom. The reaction is written

{7}N<14> + {2}He<4> --> {1}H<1> + {8}O<17>.

The number at the lower left of each element symbol in the above
reaction is the proton number. This number determines the basic chemical
identity of an atom, and it is this number scientists must change before
one element can be transformed into another. The common way to
accomplish this artificially is by bombarding nuclei with nuclear
projectiles.

Ruthe