A CENTURY OF PROGRESS 179
according to its properties. He was
wrong—the relative atomic weight
of tellurium is in fact 127.6; it is
greater than that of iodine. A
similar anomaly occurs between
potassium (weight 39) and argon
(weight 40), where argon clearly
precedes potassium in the table—
but Mendeleev was not aware of
these problems in 1869, because
argon was not discovered until
- Argon is one of the noble
gases, which are colorless, odorless,
and hardly react with other
elements. Difficult to detect, none
of the noble gases were known at
that time, so there were no spaces
for them in Mendeleev’s table. Once
argon had turned up, however,
there were several more holes to fill,
and by 1898, Scottish chemist
William Ramsay had isolated
helium, neon, krypton, and xenon.
In 1902, Mendeleev incorporated
the noble gases into his table as
Group 18, and this version of the
table forms the basis of the periodic
table we use today.
The anomaly of the “wrong”
atomic weights was solved in 1913
by British physicist Henry Moseley,
who used X-rays to determine the
number of protons in the nucleus of
each atom of a particular element.
This came to be called the atomic
number of the element, and it is
this number that determines the
element’s position on the periodic
table. The fact that atomic weights
had given a close approximation
followed from the fact that for the
lighter elements, the atomic weight
is roughly (though not exactly)
twice the atomic number.Using the table
The periodic table of the elements
may look like just a cataloguing
system—a neat way of ordering
the elements—but it has far greater
importance in both chemistry
and physics. It allows chemists
to predict the properties of an
element, and to try variations
in processes; for example, if a
particular reaction does not work
with chromium, perhaps it works
with molybdenum, the element
below chromium in the table.
The table was also crucial in
the search for the structure of the
atom. Why did the properties of
elements repeat in these patterns?
Why were the Group 18 elements
so unreactive, while the elements
in the groups on either side were
the most reactive of all? Such
questions led directly to the picture
of the structure of the atom that has
been accepted ever since.
Mendeleev was to some extent
lucky to have been credited for his
table. Not only did he publish his
ideas after Béguyer and Newlands,
but also German chemist Lothar
Meyer, who plotted atomic weight
against atomic volume to show
the periodic relationship between
elements, was ahead of him, too,
publishing in 1870. As so often in
science, the time had been ripe for
a particular discovery, and several
people had reached the same
conclusion independently, without
knowing about each other’s work. ■We must expect the discovery
of elements analogous to
aluminum and silicon—
whose atomic weight would
be between 65 and 75.
Dmitri MendeleevDmitri Mendeleev
The youngest of at least 12
children, Dmitri Mendeleev
was born in 1834 in a village
in Siberia. When his father
went blind and lost his
teaching post, Mendeleev’s
mother supported the family
with a glass factory business.
When that burned down, she
took her 15-year-old son across
Russia to St. Petersburg to
receive a higher education.
In 1862, Mendeleev
married Feozva Nikitichna
Leshcheva, but in 1876 he
became obsessed with Anna
Ivanova Popova, and married
her before his divorce from
his first wife was final.
In the 1890s, Mendeleev
organized new standards
for producing vodka. He
investigated the chemistry
of oil, and helped to set up
Russia’s first oil refinery.
In 1905, he was elected a
member of the Royal Swedish
Academy of Science, who
recommended him for a Nobel
Prize, but his candidacy was
blocked, possibly due to his
bigamy. The radioactive
element 101 mendelevium
is named in his honor.Key work1870 Principles of Chemistry