Chemistry Reference
In-Depth Information
As compared to the modern periodic table, the
Attempted System
is rotated 90
clockwise. The periods are lying vertically, rather than horizontally, and similar
elements are grouped in horizontal rows, rather than vertical columns. Notice also
that the rows of alkali metals and halogens are adjacent to one another (the noble
gases were not yet discovered in 1869). Finally, many of the problematical elements
are simply grouped on the right-hand side of the table in no particular order
whatsoever.
When inspecting Mendeleev
s
Attempted System
more closely, one cannot help
it to be struck by the following intriguing fact: Normally, when reading from top to
bottom, and from left to right, one should obtain an ever increasing atomic weight
sequence.
7
It appears however that this sequence gets interrupted twice. First, when
passing from In (75.6, third column, bottom row) to Ti (50, fourth column, upper
row), and a second time, when moving from Th (118, fourth column, bottom row)
to Zr (90, fifth column, upper row).
8
One possible way of restoring the sequence
consists in eliminating a total of seven elements from the system—namely Er, Yt,
In, Ce, La, Di, and Th.
9
Indeed, if such is the case, the sequence passes from the
unknown element with an atomic weight of 45 to Ti with an atomic weight of
50, and from Sr with an atomic weight of 87.6 to Zr with an atomic weight of 90 - in
accordance with the gradual increase in atomic weight.
It thus appears that “seven little studied elements remained outside of the table”.
10
This septuplet of homeless elements consisted of indium (In), thorium (Th) and
the five rare earths—erbium (Er), yttrium (Yt), cerium (Ce), lanthanum (La), and
didymium (Di).
11
Question marks and wrong atomic weights reigned in the last rows
of Mendeleev
'
s system, and a new place had to be found for the homeless septuplet.
We will call this problematic accommodation issue the
rare
-
earth crisis
.
'
7
It would be scientifically more correct to use the term
relative atomic mass
. In this chapter
however, the historical term
atomic weight
will be used throughout.
8
Notice that the atomic weight sequence is violated two more times by the inversion of tellurium
(Te) and iodine (I), and by the insertion of gold (Au) and bismuth (Bi).
9
Yt is an old symbol for yttrium (Y). Di, on the other hand, was the symbol for the element
didymium, which later turned out to be a mixture of praseodymium (Pr) and neodymium (Nd).
10
Quoted from Brooks, N. M. “Developing the Periodic Law: Mendeleev
'
s Work During 1869-
1871.”
Foundations of Chemistry
(2002): 129.
11
It must be remarked that terbium, another rare-earth element, was already known at the time
when Mendeleev drew up his first periodic system, but Dmitrii Ivanovich decided to not include
this element—following as a matter of fact Bunsen and Bahr
'
s advice. Terbia had been discovered
as early as 1843 by Mosander, but Mendeleev was not convinced that he was dealing with a
genuine element. He therefore wrote the symbol of terbium (Ter.) in the marginalia of a piece of
scrap paper when he was composing the
Attempted System
, but directly underneath this symbol
Mendeleev scribbled that “it does not exist according to Bunsen” (
). In
writing this sentence, Mendeleev was in all probability referring to Bahr, J. F., and Bunsen, R.
“Ueber Erbinerde Und Yttererde.”
Annalen der Chemie und Pharmacie
137 (1866a): 1-33. See
also Bahr, J. F., and R. Bunsen. “Ueber Erbinerde Und Yttererde.”
Chemisches Central
-
Blatt
11, no. 8 (1866b): 118-125, and Bahr, J. F., and R. Bunsen. “Ueber Erbinerde Und Yttererde.”
Zeitschrift fur Chemie
9 (1866c): 72-77.
не существует по бунзену
