simply called the rare earths, al-
though strictly the ‘earths’ are their
oxides. Nor are they particularly rare:
they occur widely, usually together.
All are silvery very reactive metals.
The f-electrons do not penetrate to
the outer part of the atom and there
is no f-orbital participation in bond-
ing (unlike the d-orbitals of the main
*transition elements) and the el-
ements form few coordination com-
pounds. The main compounds
contain M3+ions. Cerium also has the
highly oxidizing Ce4+state and eu-
ropium and ytterbium have a M2+
state.
The 4f orbitals in the atoms are not
very effective in shielding the outer
electrons from the nuclear charge. In
going across the series the increasing
nuclear charge causes a contraction
in the radius of the M3+ion – from
0.1061 nm in lanthanum to
0.0848 nm in lutetium. This effect,
the lanthanoid contraction (or lan-
thanide contraction), accounts for the
similarity between the transition el-
ements zirconium and hafnium.lanthanonsSee lanthanoids.
lanthanumSymbol La. A silvery
metallic element belonging to group
3 (formerly IIIA) of the periodic table
and often considered to be one of the
*lanthanoids; a.n. 57; r.a.m. 138.91;
r.d. 6.146 (20°C); m.p. 921°C; b.p.
3457 °C. Its principal ore is bastnasite,
from which it is separated by an ion-
exchange process. There are two nat-
ural isotopes, lanthanum–139 (stable)
and lanthanum–138 (half-life
1010 –10^15 years). The metal, being py-
rophoric, is used in alloys for lighter
Ûints and the oxide is used in some
optical glasses. The largest use of lan-
thanum, however, is as a catalyst in
cracking crude oil. Its chemistry re-
sembles that of the lanthanoids. The
element was discovered by Carl
Mosander (1797–1858) in 1839.lanthanons 316l
A
- Information from the WebElements site
lapis lazuliA blue rock that is
widely used as a semiprecious stone
and for ornamental purposes. It is
composed chieÛy of the deep blue
mineral lazuriteembedded in a ma-
trix of white calcite and usually also
contains small specks of pyrite. It oc-
curs in only a few places in crys-
talline limestones as a contact
metamorphic mineral. The chief
source is Afghanistan; lapis lazuli
also occurs near Lake Baikal in
Siberia and in Chile. It was formerly
used to make the artists’ pigment ul-
tramarine.Laporte selection ruleA selec-
tion rule in atomic spectra stating
that spectral lines associated with
electric-dipole radiation must arise
from transitions between states of
opposite parity. The Laporte selection
rule was discovered by O. Laporte in
1924 and was explained by the appli-
cation of group theory to the *quan-
tum mechanics of atoms. In the case
of magnetic-dipole and quadrupole
radiation the selection rule for spec-
tral lines is the opposite of the La-
porte rule, i.e. transitions are only
allowed between states of the same
parity in these cases.
Larmor precession A precession
of the motion of charged particles in
a magneticÜeld. It wasÜrst deduced
in 1897 by Sir Joseph Larmor
(1857–1942). Applied to the orbital
motion of an electron around the nu-
cleus of an atom in a magneticÜeld
ofÛux density B, the frequency of
precession is given by eB/4πmvμ,
where e and m are the electronic
charge and mass respectively, μis
the permeability, and v is the velocity
of the electron. This is known as the
Larmor frequency.laser(light ampliÜcation by stimu-