Chemistry Reference
In-Depth Information
11.1.3 Wrong Atomic Weights and Erroneous Valencies
Due to the recent discoveries of the rare-earth elements, most of their chemical and
physical properties were still shrouded in mist at the beginning of 1869, and
Mendeleev had to manage with the limited information he had at his disposal.
At the time, Mendeleev still adhered to the old Berzelian atomic weights for the
rare-earth elements. Not one of these values corresponded to the real atomic
weights however, as they were based on the erroneous assumption that most rare-
earth elements were
bivalent
instead of
trivalent
.
15
Their oxides were thus
represented by the formula RO (with the higher oxide of cerium denoted as R
2
O
3
).
With the wrong atomic weights at hand, Mendeleev naturally failed to accom-
modate the rare earths, and he felt obliged to position them at the periphery of his
Attempted System
. If Mendeleev was to accommodate the rare earths properly, he
would have to change their valency from 2 to 3. Even though Mendeleev would be
the first in proposing this modification of valency number, he only did so at the end
of the first half of 1870. Before that time (that is, during the first period from 1869
till 1870), Mendeleev continued to look upon the rare-earth elements as being
bivalent and he used the wrong atomic weights throughout.
This notwithstanding, Mendeleev certainly had doubts about the position of the
rare-earth elements from the very outset. As he admitted in his 1869 article
On the
Correlation between the Properties of the Elements and Their Atomic Weights
:
With respect to the position of some elements, there exists, quite understandably, complete
uncertainty. In particular, this holds for those elements that are little studied and whose
correct atomic weight has hardly been established with any certainty. Among these are, for
example, yttrium, thorium, and indium.
16
11.1.4 The Companions of Cerium
When Lavoisier defined a chemical element in 1789, 26 elements were actually
known. Eighty years later, at the onset of 1869, a total of 36 elements had been
added to the list. More and more chemists consequently felt the need for a
systematic organisation. But instead of building a periodic table, as Mendeleev
did in 1869, they felt inclined to group together elements with similar physical and
chemical characteristics—ending up with a network of small, so-called
natural
groups
. Well-known examples were the highly reactive halogens (F, Cl, Br, I) and
the silver coloured and water-reactive alkali metals (Li, Na, K, Rb, Cs).
15
The atomic weight
AW
of an element
X
is related to its valency
V
via the general formula
AW
(
X
)
¼
EW
(
X
)
xV
(
X
), with
EW
the equivalent weight of
X
.
16
Mendeleev, D. I. “On the Correlation between the Properties of the Elements and Their Atomic
Weights.” In
Mendeleev on the Periodic Law
,
Selected Writings
,
1869
-
1905
, ed. William
B. Jensen, 30. Mineola, New York: Dover Publications, Inc., 2002.
