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Periodic Table，希望大家喜欢!
托福阅读TPO16阅读原文
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.
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