O

occlusion1.The trapping of small
pockets of liquid in a crystal during
crystallization. 2.The absorption of
a gas by a solid such that atoms or
molecules of the gas occupy intersti-
tial positions in the solid lattice. Pal-
ladium, for example, can occlude
hydrogen.

ochre A yellow or red mineral form
of iron(III) oxide, Fe 2 O 3 , used as a pig-
ment.

octadecanoateSee stearate.

octadecanoic acidSee stearic
acid.

octadecenoic acidA straight-
chain unsaturated fatty acid with the
formula C 17 H 33 COOH. Cis-octadec-9-
enoic acid (see oleic acid) has the for-
mula CH 3 (CH 2 ) 7 CH:CH(CH 2 ) 7 COOH.
The glycerides of this acid are found
in many natural fats and oils.

octahedralSee complex.

octahydrateA crystalline hydrate
that has eight moles of water per
mole of compound.

octaneA straight-chain liquid
*alkane, C 8 H 18 ; r.d. 0.7; m.p.
–56.79°C; b.p. 125.66°C. It is present
in petroleum. The compound is iso-
meric with 2,2,4-trimethylpentane,
((CH 3 ) 3 CCH 2 CH(CH 3 ) 2 , iso-octane). See
octane number.

octane numberA number that
provides a measure of the ability of a
fuel to resist knocking when it is
burnt in a spark-ignition engine. It is
the percentage by volume of iso-
octane (C 8 H 18 ; 2,2,4-trimethylpen-
tane) in a blend with normal heptane
(C 7 H 16 ) that matches the knocking

behaviour of the fuel being tested in

a single cylinder four-stroke engine

of standard design. Compare cetane

number.

octanitrocubane See cubane.

octanoic acid (caprylic acid)A

colourless liquid straight-chain

saturated *carboxylic acid,

CH 3 (CH 2 ) 6 COOH; b.p. 239.3°C.

octavalentHaving a valency of

eight.

octaveSee law of octaves.

octetA stable group of eight elec-

trons in the outer shell of an atom

(as in an atom of a noble gas).

octogenSee hmx.

octupoleA set of eight point

charges that has zero net charge and

does not have either a dipole mo-

ment or a quadrupole moment. An

example of an octupole is a methane

molecule (CH 4 ). Octupole interactions

are much smaller than quadrupole

interactions and very much smaller

than dipole interactions.

ohmSymbol Ω. The derived *SI

unit of electrical resistance, being

the resistance between two points on

a conductor when a constant poten-

tial difference of one volt, applied be-

tween these points, produces a

current of one ampere in the conduc-

tor. The former international ohm

(sometimes called the ‘mercury

ohm’) was deÜned in terms of the re-

sistance of a column of mercury. The

unit is named after Georg Ohm

(1787–1854).

oilAny of various viscous liquids