Dictionary of Chemistry [6th Ed.]

gree of overlap between the three

concepts.

layer latticeA crystal structure in

which the atoms are chemically

bonded in plane layers, with rela-

tively weak forces between atoms in

adjacent layers. Graphite and micas

are examples of substances having

layer lattices (i.e. they are *lamellar

solids).

lazuriteSee lapis lazuli.

LCAO(linear combination of atomic

orbitals)A molecular *orbital

formed by the linear combination of

atomic orbitals. The LCAO approxi-

mation arises because with an elec-

tron that is very close to a nucleus,

the potential energy of the electron

is dominated by the interaction be-

tween the electron and the nucleus.

Thus, very close to the nucleus of an

atom, A, in a molecule, the *wave

function of the molecule is very simi-

lar to the wave function of the atom

A. The LCAO approximation shows

the increase in electron density asso-

ciated with chemical bonding. The

LCAO method takes account of the

symmetry of the molecule using sym-

metry-adapted linear combinations

(SALC).

LCPSee liquid-crystal polymer.

L-dopaSee dopa.

L–D process See basic-oxygen

process.

leachingExtraction of soluble com-

ponents of a solid mixture by perco-

lating a solvent through it.

leadSymbol Pb. A heavy dull grey

soft ductile metallic element belong-

ing to *group 14 (formerly IVB) of

the periodic table; a.n. 82; r.a.m.

207.19; r.d. 11.35; m.p. 327.5°C; b.p.

1740 °C. The main ore is the sulphide

galena (PbS); other minor sources in-

clude anglesite (PbSO 4 ), cerussite

(PbCO 3 ), and litharge (PbO). The

metal is extracted by roasting the ore

to give the oxide, followed by reduc-

tion with carbon. Silver is also recov-

ered from the ores. Lead has a variety

of uses including building construc-

tion, lead-plate accumulators, bullets,

and shot, and is a constituent of such

alloys as solder, pewter, bearing met-

als, type metals, and fusible alloys.

Chemically, it forms compounds

with the +2 and +4 oxidation states,

the lead(II) state being the more sta-

ble.

A

• Information from the WebElements site

lead(II) acetateSee lead(ii)

ethanoate.

lead–acid accumulatorAn accu-

mulator in which the electrodes are

made of lead and the electrolyte con-

sists of dilute sulphuric acid. The

electrodes are usually cast from a

lead alloy containing 7–12% of anti-

mony (to give increased hardness and

corrosion resistance) and a small

amount of tin (for better casting

properties). The electrodes are coated

with a paste of lead(II) oxide (PbO)

andÜnely divided lead; after inser-

tion into the electrolyte a ‘forming’

current is passed through the cell to

layer lattice 320

l

2e

PbPbPb2+2+2+ PbPbPb2+2+2+

2H2H2H 222 SOSOSO 444 + 4H+ 4H+ 4H+++2O2O2O2–2–2–

2PbSO2PbSO2PbSO 444

2H2H2H 222 OOO

sulphuric acid

2SO2SO2SO 444 2–2–2–

lead (Pb) lead(IV) oxide

PbO 2

Lead-acid accumulator