5. NIELS BOHR AND HENRY MOSELEY 1 Bohr’s Atomic Theory
5.5 Intergroup Accommodation Methodologies
would have been bombarded too and these could have formed some rare-earth isotopes.
In the year 1932, theneutronwas discovered by Sir James Chadwick (1891–1974) as a new, neutral elementary particle which constituted the atomic nucleus (Chadwick, 1932). Since it was not electrically charged, it proved very useful to penetrate the nuclei of other atoms in order to form new nuclides. Physicists soon discovered the processes of fission when they started bombarding certain uranium isotopes with neutrons.
They produced daughter nuclides ranging from zinc to gadolinium.
It thus appeared that isotopes of the element 61 could be produced during the fission process of uranium-235 as well. A number of chemists, physi- cists, and engineers studied the formation of these isotopes during the Manhattan Project in 1942. A whole range of new techniques had to be developed in order to separate the different nuclides. The ion exchange chromatographic methods proved very valuable. Polymers were used as ion exchangers. The American chemists Jacob Akiba Marinsky (1918–
2005), Lawrence Elgin Glendenin (1918–2008) and Charles Dubois Coryell (1912–1971) working at Oak Ridge National Laboratory (ORNL) in Tenessee soon succeeded in separating the different lanthanide nuclides.
They also obtained some fractions which contained element number 61.
In 1945, a millionth of a gram was obtained of the isotopes with mass number 147 and 149. These isotopes had been generated by two different methods: nuclear fission of uranium and bombardment of neodymium with neutrons. Finding a name for element 61 proved however as difficult as the production of its isotopes. It was Coryell’s wife Grace Mary who proposed to name the element promethium (symbol: Pm). The mythical Prometheus, one of the titans in Greek mythology, had stolen the fire from the gods for the benefit of mankind. Zeus decided to punish Prometheus for his acts and he chained him to a big rock. An eagle came to visit him each day and always ate a small piece of his liver, just small enough so that it could grow again in one day. The choice of their name was twofold.
For one thing, it referred to the harnessing of nuclear energy by humans in order to synthesize new elements. On the other hand, the name warned everybody for the ‘‘eagle of war.’’ The discovery of promethium was first announced at an American Chemical Society Meeting in New York in September 1947 (Marinsky et al., 1947). On July 28, 1948, a total of 3 mg of yellow promethium chloride and pink promethium nitrate were exhibited before the American Chemical Society.
completely unconnected to the other groups. This was accomplished by accommodating the rare earths in between two groups of the periodic table. The rare-earth elements thus showed some analogy with the transi- tion metals in view of the fact that both types of elements were separated from the rest of the system and that both formed in a sense the transition between two main groups of Mendeleev’s system. This type of placement was in complete agreement with Bohr’s quantum theory of the atom, and consequently became the preferred methodology in the twentieth century.
A particularly interesting classification was the one with horizontal groups and vertical periods proposed by the Danish thermochemist Hans Peter Jrgen Julius Thomsen (1826–1909) in 1895 (Figure 23) (Thomsen, 1895). Such a pyramidal/ladder form representation had already been proposed by the English scientist Thomas Bayley in 1882 (Figure 24), but
FIGURE 23 Julius Thomsen’s pyramidal periodic table (1895). Reproduced from Thomsen (1895).
Thomsen’s system differed from Bayley’s pyramid in an important way.
Both tables consisted of three main parts. The first part contained the elements of the short periods (Li–F and Na–Cl); the second part included the long periods of 17 elements (K–Br and Rb–J); and the third section covered the remaining 31 elements from cesium to uranium. Analogous elements were connected by lines and due to the existence of odd and even series, most elements were related to a pair of elements. Thus Na was related
I II III IV V
K Rb
Cs
Li Na
Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga –
–
– – – – – –
– –″
″″
″
″″ As
Se Br Mg
Al Si P S Cl Be
B C N O F H
Sr Y Zr Nb Mo
Ru Rh Pd Ag Cd In Sn Sb Te I
Ba Ce La Di
Er
Ta W
Os Ir Pt Au Hg Tl Pb Bi
Th
FIGURE 24 Bayley’s pyramidal periodic table (1882). Reproduced from Bayley (1882).
with K and Cu (and therefore also with Rb and Ag). Some elements remained completely unconnected. These were the transition metal triads {Fe, Co, Ni}, {Ru, Rh, Pd}, and {Os, Ir, Pt} which represented a transition from the odd to the even series. The important difference between the two tables is that Thomsen connected the elements Rb–Ag with only one element, instead of two as Bayley did. The consequences were explained by Thomsen in his paper:
Just as silicon in the first group shows similarities with titanium on the one hand and with germanium on the other, there exists in exactly the same way a relationship between the elements of the 2nd and 3rd groups, for example from zirconium to cerium with an atomic weight of 140 on the one hand and from zirconium to an unknown element with an atomic weight around 181 on the other. There are a large number of rare-earth elements in between these two elements which are related to one another like the central elements of the 3rd series placed between manganese and zinc.
(Thomsen, 1895; Trifonov, 1966).
Thomsen even tried to explain the analogy between the rare-earth elements on the one hand and the elements from group VIII on the other, just as Mendeleev had tried 26 year earlier (see Section 3). But the most important aspect of Thomsen’s table was not the apparent analogy with the transition metals, but the fact that the rare-earth elements did not bear any relationship with the elements of the preceding period from Rb to I (except for La, Ce and Yb). The rare earths did not belong to any of the eight groups and they were fitted in between group IV and V as an intergroup. Thomsen moreover correctly predicted the existence of a total of 15 rare earths from lanthanum to the unknown element after ytterbium. The element with atomic weight 181 did not belong to the rare-earth elements, but was a zirconium homologue. One thus starts to understand why Bohr preferred this table and why he used it during his Noble Lecture in 1922 (Figure 21) (Bohr, 1922b). The element after ytterbium was lutetium, a genuine trivalent rare earth, while element 72 clearly was a tetravalent zirconium homologue. Bohr remembered this table from his student time. A large version of Thomsen’s table hung in one of the auditoria of the Polytechnical Institute where Bohr was following the lectures on inorganic chemistry from Niels Bjerrum (1879–1958).
The sequence of rare earths in Thomsen’s table was almost correct.
He left a number of vacant spaces between Pr and Sm for element 61 (Pm), between Sm and Gd for Eu, between Tb and Er and between Er and Tm for Dy/Ho, and after Yb for Lu and Hf. Bohr was not the first to grasp the advantages of Thomsen’s intergroup layout. Both Richards (1898) and the Australian chemist Steele (1901) adhered to the intergroup methodology.
It must also be noted that Thomsen’s system was not the first so-called
inverted system, with horizontal groups and vertical periods. The chemist Henry Bassett (1892) had proposed a similar system and he also adhered to the intergroup methodology (Figure 25). The rare-earth elements thus formed a separate group and consisted of 18 elements. If these systems would be reverted again, vertical groups and horizontal periods would be obtained.
Alfred Werner (1866–1919) was the first to publish a long form table according to the intergroup methodology (Figure 26) (Werner, 1905a,b).
The rare-earth elements (La–Yb) were isolated from the other elements and formed an intergroup between group II and group III. Lanthanum did not belong to the same group as scandium and yttrium. Lutetium on
FIGURE 25 Bassett’s periodic table (1892). Reproduced from Bassett (1892).
the other hand, according to Werner’s table, was not a rare-earth element, but a transition metal (see also Section 7). Werner was moreover the first to suggest the existence of another intergroup of heavier elements, below and analogous to the rare-earth elements. This idea was taken up by Glenn Seaborg and is now known as the actinide hypothesis (see also Section 6). In Werner’s system, the pair praseodymium–neodymium has been arranged according to decreasing atomic weight: Werner placed neodymium (143.6) before praseodymium (140.5). The reason given by Werner for this proposal was the similarity in color of the hydrated cobalt (II) and neodymium(III) salts which are violet on the one hand, and the similarity in color of the hydrated nickel(II) and praseodymium(III) salts which are green on the other hand. The order cobalt–nickel in the periodic table suggested Werner to choose the order neodymium–praseodymium as well. Notice that the pair cobalt–nickel is an example of the deviation from increasing atomic weight in the periodic table. Baur (1911) published a similar table, but he included the rare earths between group IV and V (except La and Ce, see Figure 27). The best representation, according to the authors’ opinion, was the left-step periodic table which had been devised in the period 1927–1929 by the French engineer, inventor and biologist Charles Janet (1849–1932) (see Section 7). A remarkable periodic system is the circular system of Janet (Janet, 1929), which was based on his left-step table (Figure 28). The advantage of this representation is its compactness. Janet’s table gives equal value to all the elements, including
FIGURE 27 Baur’s periodic table (1911). Reproduced from Baur (1911).
the rare earths. Notice that Janet places lutetium and not lanthanum below yttrium (see Section 7).
A survey of about 100 educational and professional textbooks in the field of descriptive inorganic chemistry was entailed by one of the authors (P.T.) in order to investigate the popularity of the three types of accom- modation. A total of 92 chemistry textbooks for both undergraduate and graduate students from the period 1846–1963 was selected at random and thoroughly investigated. Only 54 manuals out of the 92 did contain a periodic table. One handbook exhibited a spiral form of the periodic table (Shepard, 1886), but all other textbooks represented short and long forms of the periodic table. An important observation rested on the fact that almost all textbooks adhered to one of two possible accommodation FIGURE 28 Modified circular system of Janet (1929). Reproduced from Stewart (2007), with permission of Springer.
methodologies. Thus 40 textbooks placed the rare-earth elements accord- ing to a homologous accommodation, and the other 14 adhered to the intraperiodic accommodation methodology. The first intergroup accom- modation only appeared in 1946 (Yost and Russell, 1946). Indeed, the well known placement of both lanthanides and actinides underneath the main body of the ‘‘modern’’ periodic table became only popular after the Second World War.