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新托福TPO16阅读原文(二)：Development of the Periodic Table
TPO16-2：Development of the Periodic Table
The periodic table is a chart that reflects the periodic recurrence of chemical and
physical properties of the elements when the elements are arranged in order of
increasing atomic number (the number of protons in the nucleus). It is a monumental
scientific achievement, and its development illustrates the essential interplay between
observation, prediction, and testing required for scientific progress. In the 1800's
scientists were searching for new elements. By the late 1860's more than 60 chemical
elements had been identified, and much was known about their descriptive chemistry.
Various proposals were put forth to arrange the elements into groups based on
similarities in chemical and physical properties. The next step was to recognize a
connection between group properties (physical or chemical similarities) and atomic
mass (the measured mass of an individual atom of an element). When the elements
known at the time were ordered by increasing atomic mass, it was found that
successive elements belonged to different chemical groups and that the order of the
groups in this sequence was fixed and repeated itself at regular intervals. Thus when
the series of elements was written so as to begin a new horizontal row with each alkali
metal, elements of the same groups were automatically assembled in vertical columns
in a periodic table of the elements. This table was the forerunner of the modern table.
When the German chemist Lothar Meyer and (independently) the Russian
Dmitry Mendeleyev first introduced the periodic table in 1869-70, one-third of the
naturally occurring chemical elements had not yet been discovered. Yet both chemists
were sufficiently farsighted to leave gaps where their analyses of periodic physical
and chemical properties indicated that new elements should be located. Mendeleyev
was bolder than Meyer and even assumed that if a measured atomic mass put an
element in the wrong place in the table, the atomic mass was wrong. In some cases
this was true. Indium, for example, had previously been assigned an atomic mass
between those of arsenic and selenium. Because there is no space in the periodic table
between these two elements, Mendeleyev suggested that the atomic mass of indium
be changed to a completely different value, where it would fill an empty space
between cadmium and tin. In fact, subsequent work has shown that in a periodic table,
elements should not be ordered strictly by atomic mass. For example, tellurium comes
before iodine in the periodic table, even though its atomic mass is slightly greater.
Such anomalies are due to the relative abundance of the "isotopes" or varieties of each
element. All the isotopes of a given element have the same number of protons, but
differ in their number of neutrons, and hence in their atomic mass. The isotopes of a
given element have the same chemical properties but slightly different physical
properties. We now know that atomic number (the number of protons in the nucleus),
not atomic mass number (the number of protons and neutrons), determines chemical
behavior.
Mendeleyev went further than Meyer in another respect: he predicted the
properties of six elements yet to be discovered. For example, a gap just below
aluminum suggested a new element would be found with properties analogous to
those of aluminum. Mendeleyev designated this element "eka-aluminum" (eka is the
Sanskrit word for "next") and predicted its properties. Just five years later an element
with the proper atomic mass was isolated and named gallium by its discoverer. The
close correspondence between the observed properties of gallium and Mendeleyev’s
predictions for eka-aluminum lent strong support to the periodic law. Additional
support came in 1885 when eka-silicon, which had also been described in advance by
Mendeleyev, was discovered and named germanium.
The structure of the periodic table appeared to limit the number of possible
elements. It was therefore quite surprising when John William Strut (Lord Rayleigh,
discovered a gaseous element in 1894 that did not fit into the previous classification
scheme. A century earlier, Henry Cavendish had noted the existence of a residual gas
when oxygen and nitrogen are removed from air, but its importance had not been
realized. Together with William Ramsay, Rayleigh isolated the gas (separating it from
other substances into its pure state) and named it argon. Ramsay then studied a gas
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