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678 Periodic Table (Chemistry)

lanthanide, rare earth, or inner transition series. These ble. The fundamental correctness of the scheme developed
elements are similar and exhibit an even more gradual by Mendeleev and his contemporaries has been verified
change in properties than do the transition metals. For this through physical measurements and the computations of
reason, and for reasons related to atomic structure, the quantum mechanics. Today, the reason for the periodicity
lanthanides are grouped together in a row along the bot- of the elements is well established; the explanation lies in
tom of the table, with the understanding that they really the structure of the atom. Therefore, a discussion of tabular
fit between lanthanum (La) and hafnium (Hf). The third trends will be deferred until after this brief consideration
transition series follows, and the row is completed with of atomic structure and, in particular, the arrangement of
the main group elements. electrons within atoms.
The seventh period begins, as do others, with an al-
kali metal and an alkaline earth. Thorium (Th) begins
A. Radioactivity, Isotopes, and Atomic Mass
a series of 14 actinide elements corresponding to the
actinides. All of these elements are radioactive, and those The contemporary conception of atomic structure is a
beyond uranium (U) do not occur in nature, at least not product of the 20th century. Shortly before the turn of
in appreciable quantities. Rather, they are all artificially the century, a number of important experimental discov-
made via nuclear reactions. They are among the products eries were made, chiefly in Europe. In late 1895, Wilhelm
of modern atomic research, and their names refer to the R¨ontgen (1845–1923) discovered X-rays in his W¨urzburg
places (Berkeley and California) and the people (Fermi laboratory. Several months later, while investigating what
and Lawrence) involved. he believed to be the same phenomenon, the French physi-
cist Henri Becquerel (1852–1908) first observed natu-
B. The New Discoveries ral radioactivity. In 1897, J. J. Thompson (1856–1923)
of Cambridge University detected, in the rays emitted
The newest and heaviest elements start a fourth transition
from the cathode of a partially evacuated gas discharge
series called the transfermium elements. These are also
tube, negatively charged particles, later called electrons,
synthetic elements created in nuclear reactions, hence they
which proved to be much less massive than hydrogen
haveveryshortlifespans.In1997,theIUPACapprovedthe
atoms. Ernest Rutherford (1871–1937) demonstrated that
names of elements 100–109; however, the elements after
radioactive emissions consist of three major types: alpha
that are still referred to by their Latinized atomic num-
particles with a charge of +2 and a mass of about 4 on the
ber, for example, element 111 (Uuu) is unununium. The
standard atomic mass scale, beta rays which are identical
obvious cumbersome nature of such names has prompted
to (fast) electrons, and gamma rays which are massless
another IUPAC committee to be formed which will soon
manifestations of electromagnetic radiation. In collabora-
approve names for elements 110 through 112. These last
tion with chemist Frederick Soddy (1877–1956), Ruther-
threeelementswerealldiscoveredattheGSI(Gesellschaft
ford went on to show that when an element emits alpha
f¨ur Schwerionenforschung) in Darmstadt, Germany, in
particles, it is spontaneously transmuted into the element
1994 and 1996. The newest element is ununoctium (Uuo,
lying two places to the left in the periodic table. Thus,
118). It was created at Berkeley in 1999 and, due to rapid
uranium (U) becomes thorium (Th) after giving off alpha
(0.12 msec) alpha decay, led to the first creation of element
radiation. On the other hand, loss of beta “particles” (beta
116. This further decayed into a previously undiscovered
rays) transforms the emitting element into the element sit-
isotope of element 114 [a different isotope of element
uated immediately to its right [thorium, for example, beta
114 had been informally reported by Russian scientists
decays to protactinium (Pa)].
at Dubna (Joint Institute for Nuclear Research) in 1999].
The process of radioactive decay led to the conclusion
Due to the short lifespan of many of these elements, much
that atomic mass must not be a singularly identifying prop-
of our knowledge about their properties has come from
erty of an element. Indeed, as Soddy demonstrated, not all
high-level calculations rather than experiment.
of the atoms of any given element are identical. Most ele-
ments naturally exist as a mixture of atoms with differing
III. ATOMIC STRUCTURE AND atomic masses. Soddy called these different forms iso-
ELEMENTARY PERIODICITY topes, literally emphasizing that all isotopes of the same
element belong in the “same place” in the periodic table.
One of the great achievements of modern atomic theory The atomic masses determined for the elements from com-
is the way in which it accounts for the periodic properties bining masses, gas densities, or other traditional chemi-
of the elements—the similarities in chemical and physi- cal measurements are, in fact, weighted averages of the
cal properties within a family and the gradual changes in isotopic masses in the naturally occurring mixture. Thus,
properties as one moves down or across the periodic ta- hydrogen consists almost exclusively of atoms of mass