Dictionary of Chemistry [6th Ed.]

This can be made by neutralizing
hydrochloric acid with strontium
carbonate, oxide, or hydroxide.
Strontium chloride is used for mili-
taryÛares.

strontium hydrogencarbonate
(strontium bicarbonate) A com-
pound, Sr(HCO 3 ) 2 , which is stable
only in solution. It is formed by the
action of carbon dioxide on a suspen-
sion of strontium carbonate in water.
On heating, this process is reversed.

strontium oxide (strontia) A white
compound, SrO; r.d. 4.7; m.p. 2430°C,
b.p. 3000°C. It can be prepared by the
decomposition of heated strontium
carbonate, hydroxide, or nitrate, and
is used in the manufacture of other
strontium salts, in pigments, soaps
and greases, and as a drying agent.

strontium sulphate A white solid,
SrSO 4 ; r.d. 3.96; m.p. 1605°C. It can
be made by dissolving strontium
oxide, hydroxide, or carbonate in sul-
phuric acid. It is used as a pigment in
paints and ceramic glazes and to pro-
vide a red colour inÜreworks.

structural formulaSee formula.

structural isomerism See iso-
merism.

structure factor A quantity de-
noted Fhkl, where h, k, and l are the
Miller indices of the crystal, which
occurs in *X-ray crystallography and
other experiments involving scatter-
ing in crystals. Fhklis deÜned by the
equation:

Fhkl= ∑

i

fiexp[2πi(hxi+ kyi+ lzi)],

where the sum is over all atoms of
the unit cell and fiis the scattering
factorfor atom i deÜned by:

fi= 4π 0

∞

(ρsinkr/kr)r^2 dr

Here k = 4πsinθ/λ, where θis the
Bragg angle (see bragg’s law), λis the
wavelength of the X-rays, and ρis the
electron density distribution of the

atom i. The structure factor is used in

Patterson synthesis.

strychnineA colourless poisonous

crystalline alkaloid found in certain

plants.

styrene See phenylethene.

subjacent orbitalsThe next-to-

highest occupied molecular orbital

(NHOMO) and the second-lowest un-

occupied molecular orbital (SLUMO).

In certain cases these subjacent or-

bitals are signiÜcant in *frontier-

orbital theory.

sublimateA solid formed by subli-

mation.

sublimationA direct change of

state from solid to gas.

submillimetre waves Electromag-

netic radiation with wavelengths

below one millimetre (and therefore

frequencies greater than 300 giga-

hertz), extending to radiation of the

far infrared. A source of submilli-

metre radiation is a medium pres-

sure mercury lamp in quartz. Sub-

millimetre waves can be detected by

a *Golay cell.

subshellSee atom.

substantive dyeSee dyes.

substantivityThe afÜnity of a dye

for its substrate.

substituent1.An atom or group

that replaces another in a substitu-

tion reaction. 2.An atom or group

regarded as having replaced a hydro-

gen atom in a chemical derivative.

For example, dibromobenzene

(C 6 H 4 Br 2 ) is a derivative of benzene

with bromine substituents.

substitute natural gasSee sng.

substitution reaction (displace-

ment reaction) A reaction in which

one atom or molecule is replaced by

another atom or molecule. See elec-

507 substitution reaction

s