Threshold 3—Making Chemical Elements .......................................

LECTURE iv

[Russian chemist Dmitrii Mendeleev] found that the elements known

in his time could be arranged in such a way that the vertical columns

contained elements with similar features, features such as differing

degrees of chemical reactivity. And he also showed something else.

These features recurred at surprisingly regular intervals.

M

ost of the chemical elements from which we are constructed were
formed in dying stars. So stars laid the foundations for a new,
chemical, level of complexity. The chemical elements are the 92
most elementary forms of matter. The modern understanding of chemical
elements really goes back to the work of Russian chemist Dmitrii Mendeleev
(1834–1907).

In 1869, Mendeleev did for the chemical elements what Hertzsprung and
Russell had done for stars. He arranged all elements known in his time in
a way that revealed new and hitherto unsuspected regularities. He arranged
them in rows according to their atomic weight. (Modern versions of his
table arrange them by their “atomic number,” the number of protons in their
nuclei.) He arranged them in columns by their chemical properties and was
able to show that similar properties recur at regular intervals. Remarkably,
this enabled him to accurately predict the chemical properties of some
elements that had not yet been discovered. Mendeleev’s table is known as
the “periodic table of the elements.”

How were the different elements created? Most elements were not formed in
the big bang because, by the time protons and neutrons had been created—
within seconds of the big bang—temperatures were too low to forge
elements heavier than hydrogen and helium. Temperatures high enough to
forge heavier elements would not be re-created until the appearance of stars.
To understand how stars created heavier elements, we need to understand
their life cycles.