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The Positions of Lanthanum Actinium] and Lutetium
Lawrencium] in the Periodic Table
William B.
ensen
University of Wisconsin-Madison. Madison, WI 53706
The deba te as to whether lanthanum and actinium or lu-
tetium (and, more recently, lawrencium) should be placed in
erouo IIIB of the neriodic table alone with scandium and
vt
.
trium (or rquiv:ilintly, in electronic tir rns, which pair ihod d
he considered as he iirsr memhrrs o ft he d block ior periuds
6 and 7) has a long, but poorly publicized, history.
r
quite
some time i t has been known tha t yttrium, and, to a lesser
degree, scandium are closer in their chemical properties to
lutetium and the other heavy rare earths than they are to
lanthanum 1,2)and on this basis alone a number of chemists
in the 1920's and 1930's assigned lutetium rather than lan-
thanum
to
erouo IIIB 3).The current consensus. which olaces
.
lanrhanum and actinium in this position instead, appears to
have evolved during the 194O's a h , : with the u s e d oeriodic
tahles based on electronic configurk ion and t he concept of
th e differentiating electron.
Early spectroscopic work on t he rare earths seemed to in-
dicate tha t the ground states of their atoms had, with few
exceptions, an electronic configuration of the form [Noble
Gas](n
-
2)fx-'(n l)d'ns2. Indeed, this was thought to
conform to the ideal electronic configuration for the f-block
elements in general
(2).
Thus, ytterbium was assigned the
ground state [Xe]4f135d16s2 nd lutetium the ground sta te
[Xe]4f145d16s2, esulting in a 4f differentiating electron for
lutetium and firmly establishing it as the last member of the
f-block for period 6.Barium, on the other hand, had t he con-
figuration [Xe]6s2and lanthanum t he configuration [Xe].
5d16s2, hus giving lanthanum a 5d differentiating electron
and establishing it in group IIIB as the first member of the d
block for period 6. This assignment for lanthanum appeared
to be further justified by the analogy between its configuration
and the configurations of the other members of group IIIB:
Sc = [Ar]3d14s2 nd Y = [Kr]4d15s2.
More recent spectroscopic work, however, has revised the
earlier electronic configurations 4). Only three of the rare
earths in period 6 (La, Gd, and Lu) are now known
to
have the
ground sta te [Xe]4p-'5d16s2, all of th e rest having the con-
figuration [Xe]4fx6s2. n period
7
only six of the actinides (Ac,
Pa, U Np, Cm, and Lr ) have the old configuration. Thorium
has the configuration [Rn]6d27s2 nd th e remaining eight the
configuration [Rn]5f*7s2.This strongly suggests [Noble Gas](n
- 2)fi s2ra ther than [Noble Gas](n )fX-Vn - )dlns2 as
th e ideal ground sta te configuration for th e f-block elements
in general. Ytterbium and nobelium now have the configu-
Revised version
of the currently popular
medium length
block
form
of
the periadictable.
634
Journal
of
Chemical Education
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ration [Nohle Gas](n Z)P4ns2and lutetium and lawrencium
the configuration [Nohle Gas](n 2)f14(n )d1ns2, esulting
in a d rather than an f differentiating electron for both lute-
tium and lawrencium and makine them eouallv valid candi-
.
dnws for the first memhers of the d block in periods t and 7
On the other hand. the IRn16d27s%onfirurati tn or thorium.
which no one doubts is'au )-block element with an irregula;
configuration, strongly sul,pc,rts the supposition that both
lanthanum and actinium should be cons~dered s f-t ~lo ck
I
ements with irregular configurations derived from the ideal
configuration [Nohle Gas](n 2)f1ns2. In other words, lan-
thanum and actinium should be considered the first memhen
of the f hlock (rather than Ce and Th),ytterbium and nohel-
Atomic
Radiis
A l
1.
Periodlc Trends
in
Various
P
Sum
of
the First
TWOonization
POtentiaI~*
eV)
1 9 z \ y
1 8 6 1 1 ~
17 04
versus
ium should he considered t1.e last members of the f hlock
(rather than Lu and Lr), and lutetium and lawrencium (rather
than La and Ac) shouldbe considered the first memhen of the
d hlock in periods
6
and 7 and assigned t o Group IIIB along
with scandium and yttrium.
The argument that the total (i.e., core plus valence) elec-
tronic configurations of lanthanum and actinium are closer
to those of scandium and vtt rium than are the confieurations
of lutetium and l a w r e n c h (due to their filled (n 2)f'4
suhshells) is misleadins. One must consider intraoeriod
as
well
as intragroup analog&. Thus, the remaining kine d-block
elements of period
6
(Hf Hg) all have the complete [Xe]4f14
core like lutetium and not just the [Xe] core of lanthanum.
lrtles tor the First Part
of
the d-BI(
Melting
ointb
(OK)
F m m
ef. I). Data om ref. (f8)
Volume
59
Number
8
August
1982
635
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Likewise, in oassine down the columns of the
d
hlock from
Table 2 A Comparlson 1 Vartous Prowrttes of Sc and Y versus
Ti-Z n one alk av s t: ,uniers the addition of the 4f1'su bshe ll
on passinr from period o period 6. In sho rt, if on e wishes to
usi analogies hased on trehd s in th e configurations of the
cores, eighteen of the se analogies favor th e assignment of lu-
tetium an d lawrencium to G roup IIIB an d only one favors the
assignment of lanthan um an d actinium.
Areuments of this nature were nu t forward hv Lu der in
favor of the reassignmellt of lutetium and lawrencium over
a dec ade ago bu t app ear to have gone unno ticed (5,151. Like-
wise, the Russian che mist Christyakov, using periodic trend s
in ionization potentials and atomic radii, arrived a t he same
conclusion
f a ) .
One may also ad d
to
this list trends in ionic
radii, redox poten tials, and electronegatives,
as
well as the
chemical hehavior of scandium a nd y ttrium mentioned earlier.
Th e most compelling evidence, however, has heen presen ted
by the physicis ts ( s 1 3 ). Th is includes comparisons of tren ds
in the melting points of the elem ents (1 1,J 2) , their crystal
s t ructures at room temoe rature 11). he crvstal s t ructures
of their oxides and various intermktaiiic compounds ( l l ) , he
structu res of their excited state soectra (11 ).their suoercon-
ductivit ies (10, 11), and th e s t r uku res of t'heir c ond kt iv i ty
hands as revealed by X-ray isochromats (13). All of these
properties unanimously favor the plac ement of lutetium and
lawrencium, rather than lanthanum and act inium, in group
IIIB. Some example data a re summarized in Tables
1
an d
L
In fac t, so overwhelming is the evidence for this assignmen t
that Mazurs adopted i t in the
1974
edition of his classic
monoe ranh on th e oeriodic tahle (14). However. a auick ex-
aminat ion of over gfteen freshman chemistry te kt s in d four
popular inorganic texts published since 1975 revealed that
none of them h ad revised their periodic tables 15). ndeed,
in talking with his fellow chemists, the auth or discovered tha t
none of the m was aware of th e evidence favoring th e reas-
signment of lutetium and lawrencium or indeed that there
ever was any que stion abou t their placements (a category in
which the a uthor mus t include himself until very recently).
As chemists, the periodic tahle is presumed to he-our special
province; surely its about time we pay atten tion to w hat the
physicists have to
tell us ahout its arrangement. A revised
version of the currently popular medium-length hlock form
of the table is shown above.
Literature Cited
-. . ..
New York. 1961. Chap. 2
(3)
See,
forexampie, Shemyskin,
F.
M.,Zh. Obshch. Khim.2.62 11932) andHury, C.R.,
J. Amer Chem Soc. 43,1602 (1921). Further examples can he found n reference
114).
~ ~~~
thore 1 Lu and La
Propew
Highest common
oxidation state
Precipitation of sulfate
in fractional
crystallization'
Stlllcture of metal
at r m m
temperatureb
Shlc ture of oxide
(M20s) '
Y group Y group
Y
group
Ce
group
special
hCP hCP hc p double hc p
:[AB~,.]C
NIcS ~ A B ~ ~ ~ I Cpecial
hex. CN-7
A-MIOO
Presence of low-lying No
NO
No
Yes
nonhydrogenic
I-orbitals
&Block-like structure Yes Yes Yes NO
for
conduction
bandd
Super~onduc t iv i t y~~~ NO
No
No Yes (4.gK)
Ref.
(1)
el. (17)
ORel. (11)
'Ref. (13)
~ I . 14
'
M s ~t r c hk l n ig g l l a d i~ t i o nm u l a s
(1968). His
count
far the lanthanides, however.b ifferswith eg rd o Ce fmh