ered by a monolayer and then repeat-
ing the process. This process, called
the Langmuir–Blodgett technique, en-
ables a multilayer to be built up, one
monolayer at a time. Langmuir–
BlodgettÜlms have many potential
practical applications, including insu-
lation for optical and semiconductor
devices and selective membranes in
biotechnology.
Langmuir–Hinshelwood mecha-
nism A possible mechanism for a
bimolecular process catalyzed at a
solid surface. It is assumed that two
molecules are adsorbed on adjacent
sites and that a reaction takes place
via an activated complex on the sur-
face to yield the products of the reac-
tion. This mechanism was suggested
by the US chemist Irving Langmuir
(1881–1957) in 1921 and developed
by the British chemist Sir Cyril Hin-
shelwood (1897–1967) in 1926. Some
bimolecular reactions at surfaces are
in agreement with the predictions of
this model. See also langmuir–rideal
mechanism.
Langmuir isotherm See langmuir
adsorption isotherm.
Langmuir–Rideal mechanism A
possible mechanism for a bimolecu-
lar process catalyzed at a solid sur-
face. It is envisaged that one of the
molecules is adsorbed and then re-
acts with a second molecule that is
not adsorbed. This mechanism was
suggested by the US chemist Irving
Langmuir (1881–1957) in 1921 and
developed by the British chemist Sir
Eric Rideal in 1939. The Langmuir–
Rideal mechanism is not common
but certain reactions involving hy-
drogen atoms probably occur by this
mechanism.
lanolinAn emulsion of puriÜed
wool fat in water, containing choles-
terol and certain terpene alcohols
and esters. It is used in cosmetics.
lansfordite A mineral form of
*magnesium carbonate pentahy-
drate, MgCO 3 .5H 2 O.
lanthanide contractionSee lan-
thanoids.
lanthanidesSee lanthanoids.lanthanoid contractionSee lan-
thanoids.lanthanoids(lanthanides; lan-
thanons; rare-earth elements) A se-
ries of elements in the *periodic
table, generally considered to range
in proton number from cerium
(58) to lutetium (71) inclusive. The
lanthanoids all have two outer s-
electrons (a 6s^2 conÜguration), follow
lanthanum, and are classiÜed to-
gether because an increasing proton
number corresponds to increase in
number of 4f electrons. In fact, the 4f
and 5d levels are close in energy and
theÜlling is not smooth. The outer
electron conÜgurations are as fol-
lows:
57 lanthanum (La) 5d^16 s^2
58 cerium (Ce) 4f 5 d^16 s^2 (or 4f^26 s^2 )
59 praseodymium (Pr) 4f^36 s^2
60 neodymium (Nd) 4f^46 s^2
61 promethium (Pm) 4f^56 s^2
62 samarium (Sm) 4f^66 s^2
63 europium (Eu) 4f^76 s^2
64 gadolinium (Gd) 4f^75 d^16 s^2
65 terbium (Tb) 4f^96 s^2
66 dysprosium (Dy) 4f^106 s^2
67 holmium (Ho) 4f^116 s^2
68 erbium (Er) 4f^126 s^2
69 thulium (Tm) 4f^136 s^2
70 ytterbium (Yb) 4f^146 s^2
71 lutetium (Lu) 4f^145 d^16 s^2
Note that lanthanum itself does
not have a 4f electron but it is gener-
ally classiÜed with the lanthanoids
because of its chemical similarities,
as are yttrium (Yt) and scandium (Sc).
Scandium, yttrium, and lanthanum
are d-block elements; the lanthanoids
and *actinoids make up the f-block.
The lanthanoids are sometimes315 lanthanoids
l