Dictionary of Chemistry [6th Ed.]

long period 332

l

H

H

H

N

Lone pair

considering the shapes of molecules,

repulsions between bonds and lone

pairs can be taken into account:

lone pair–lone pair >lone

pair–bond >bond–bond.

long periodSee periodic table.

Lorentz–Lorenz equation A rela-

tion between the *polarizability αof

a molecule and the refractive index

n of a substance made up of mol-

ecules with this polarizability. The

Lorentz–Lorenz equation can be writ-

ten in the form α= (3/4πN) [(n^2 –1/

(n^2 + 2)], where N is the number of

molecules per unit volume. The

equation provides a link between a

microscopic quantity (the polarizabil-

ity) and a macroscopic quantity (the

refractive index). It was derived using

macroscopic electrostatics in 1880 by

Hendrik Lorentz (1853–1928) and in-

dependently by the Danish physicist

Ludwig Valentin Lorenz also in 1880.

Compare clausius–mossotti equa-

tion.

Loschmidt’s constant (Loschmidt

number)The number of particles

per unit volume of an *ideal gas at

STP. It has the value 2.686 763(23) ×

1025 m–3and wasÜrst worked out by

Joseph Loschmidt (1821–95).

Lotka–Volterra mechanism A

simple chemical reaction mechanism

proposed as a possible mechanism of

*oscillating reactions. The process in-

volves a conversion of a reactant R

into a product P. The reactantÛows

into the reaction chamber at a con-

stant rate and the product is re-

moved at a constant rate, i.e. the re-

action is in a steady state (but not in

chemical equilibrium). The mecha-

nism involves three steps:

R + X →2X

X + Y →2Y

Y →P

TheÜrst two steps involve *auto-

catalysis: theÜrst step is catalysed by

the reactant X and the second by the

reactant Y. The kinetics of such a re-

action can be calculated numerically,

showing that the concentrations of

both X and Y increase and decrease

periodically with time. This results

from the autocatalytic action. Ini-

tially, the concentration of X is small,

but, as it increases, there is a rapid

increase in the rate of theÜrst reac-

tion because of the autocatalytic ac-

tion of X. As the concentration of X

builds up, the rate of the second reac-

tion also increases. Initially, the con-

centration of Y is low but there is a

sudden surge in the rate of step 2, re-

sulting from the autocatalytic action

of Y. This lowers the concentration of

X and slows down step 1, so the con-

centration of X falls. Less X is now

available for the second step and the

concentration of Y also starts to fall.

With this fall in the amount of Y, less

X is removed, and theÜrst reaction

again begins to increase. These

processes are repeated, leading to re-

peated rises and falls in the concen-

trations of both X and Y. The cycles

are not in phase, peaks in the con-

centration of Y occurring later than

peaks in X.

In fact, known oscillating chemical

reactions have different mechanisms

to the above, but the scheme illus-

trates how oscillation may occur.

This type of process is found inÜelds

other than chemistry; they were in-

vestigated by the Italian mathemati-

cian Vito Volterra (1860–1940) in