simply called the rare earths, although 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 bonding (unlike the d-orbitals of the main
*transition elements) and the elements form few coordination compounds. The main compounds
contain M
3+ ions. Cerium also has the
highly oxidizing Ce
4+ state and europium and ytterbium have a M
2+
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 M
3+ ion – from
0.1061 nm in lanthanum to
0.0848 nm in lutetium. This effect,
the lanthanoid contraction (or lanthanide contraction), accounts for the
similarity between the transition elements zirconium and hafnium.
lanthanons See lanthanoids.
lanthanum Symbol 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 ionexchange process. There are two natural isotopes, lanthanum–139 (stable)
and lanthanum–138 (half-life
10
10 –10
15 years). The metal, being pyrophoric, is used in alloys for lighter
Ûints and the oxide is used in some
optical glasses. The largest use of lanthanum, however, is as a catalyst in
cracking crude oil. Its chemistry resembles that of the lanthanoids. The
element was discovered by Carl
Mosander (1797–1858) in 1839.
lanthanons
316
l
A
• Information from the WebElements site
lapis lazuli A blue rock that is
widely used as a semiprecious stone
and for ornamental purposes. It is
composed chieÛy of the deep blue
mineral lazurite embedded in a matrix of white calcite and usually also
contains small specks of pyrite. It occurs in only a few places in crystalline 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 ultramarine.
Laporte selection rule A selection 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 application of group theory to the *quantum mechanics of atoms. In the case
of magnetic-dipole and quadrupole
radiation the selection rule for spectral lines is the opposite of the Laporte 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 magnetic Üeld. It was Ürst deduced
in 1897 by Sir Joseph Larmor
(1857–1942). Applied to the orbital
motion of an electron around the nucleus of an atom in a magnetic Üeld
of Ûux density B, the frequency of
precession is given by eB/4πmvµ,
where e and m are the electronic
charge and mass respectively, µ is
the permeability, and v is the velocity
of the electron. This is known as the
Larmor frequency.
laser (light ampliÜcation by stimuwww.AzShimi.ir www.AzShimi.com
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 bonding (unlike the d-orbitals of the main
*transition elements) and the elements form few coordination compounds. The main compounds
contain M
3+ ions. Cerium also has the
highly oxidizing Ce
4+ state and europium and ytterbium have a M
2+
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 M
3+ ion – from
0.1061 nm in lanthanum to
0.0848 nm in lutetium. This effect,
the lanthanoid contraction (or lanthanide contraction), accounts for the
similarity between the transition elements zirconium and hafnium.
lanthanons See lanthanoids.
lanthanum Symbol 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 ionexchange process. There are two natural isotopes, lanthanum–139 (stable)
and lanthanum–138 (half-life
10
10 –10
15 years). The metal, being pyrophoric, is used in alloys for lighter
Ûints and the oxide is used in some
optical glasses. The largest use of lanthanum, however, is as a catalyst in
cracking crude oil. Its chemistry resembles that of the lanthanoids. The
element was discovered by Carl
Mosander (1797–1858) in 1839.
lanthanons
316
l
A
• Information from the WebElements site
lapis lazuli A blue rock that is
widely used as a semiprecious stone
and for ornamental purposes. It is
composed chieÛy of the deep blue
mineral lazurite embedded in a matrix of white calcite and usually also
contains small specks of pyrite. It occurs in only a few places in crystalline 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 ultramarine.
Laporte selection rule A selection 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 application of group theory to the *quantum mechanics of atoms. In the case
of magnetic-dipole and quadrupole
radiation the selection rule for spectral lines is the opposite of the Laporte 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 magnetic Üeld. It was Ürst deduced
in 1897 by Sir Joseph Larmor
(1857–1942). Applied to the orbital
motion of an electron around the nucleus of an atom in a magnetic Üeld
of Ûux density B, the frequency of
precession is given by eB/4πmvµ,
where e and m are the electronic
charge and mass respectively, µ is
the permeability, and v is the velocity
of the electron. This is known as the
Larmor frequency.
laser (light ampliÜcation by stimuwww.AzShimi.ir www.AzShimi.com