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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