fractional distillation (fractiona-
tion)The separation of a mixture of
liquids by distillation. Effective sepa-
ration can be achieved by using a
long vertical column (fractionating
column) attached to the distillation
vessel andÜlled with glass beads.
Vapour from the liquid rises up the
column until it condenses and runs
back into the vessel. The rising
vapour in the columnÛows over the
descending liquid, and eventually a
steady state is reached in which
there is a decreasing temperature
gradient up the column. The vapour
in the column has more volatile com-
ponents towards the top and less
volatile components at the bottom.
Various fractionsof the mixture can
be drawn off at points on the col-
umn. Industrially, fractional distilla-
tion is performed in large towers
containing many perforated trays. It
is used extensively in petroleum
reÜning.fractionating columnSee frac-
tional distillation.
fractionationSee fractional dis-
tillation.
franciumSymbol Fr. A radioactive
element belonging to *group 1 (for-
merly IA) of the periodic table; a.n.
87; r.d. 2.4; m.p. 27± 1 °C; b.p.
677 ± 1 °C. The element is found in
uranium and thorium ores. All 22
known isotopes are radioactive, the
most stable being francium–223. The
existence of francium was conÜrmed
by Marguerite Perey in 1939.
A- Information from the WebElements site
Franck–Condon principleA prin-
ciple governing the intensity of tran-
sitions in the vibrational structure
during an electronic transition in a
molecule. The principle states that
since nuclei are much heavier and
move much more slowly than elec-trons (see born–oppenheimer approx-
imation), an electronic transition oc-
curs much more rapidly than the
time required for the nuclei to re-
spond to it. Therefore, in a diagram
showing the electronic states of the
molecule as a function of internu-
clear distance, the most intense elec-
tronic transition is represented by a
vertical line. For this reason a transi-
tion obeying the Franck–Condon
principle is called a vertical
transition; when it occurs the relative
positions of the nuclei remain un-
changed. The Franck–Condon princi-
ple is named after James Franck
(1882–1964), who stated it in 1925,
and Edward Condon, who formu-
lated it mathematically in terms of
quantum mechanics in 1928.
Frankland, Sir Edward (1825–99)
British organic chemist who was the
Ürst to produce organometallic com-
pounds (zinc dialkyls). He is remem-
bered as the originator of the theory
of valency and introduced a method
of writing structural formulas.Frasch process A method of ob-
taining sulphur from underground
deposits using a tube consisting of
three concentric pipes. Superheated
steam is passed down the outer pipe
to melt the sulphur, which is forced
up through the middle pipe by com-
pressed air fed through the inner
tube. The steam in the outer casing
keeps the sulphur molten in the
pipe. It was named after the German-
born US chemist Hermann Frasch
(1851–1914).Fraunhofer, Josef von
(1787–1826) German physicist, who
trained as an optician. In 1814 he ob-
served dark lines in the spectrum of
the sun (see fraunhofer lines). He
also studied diffraction.
Fraunhofer linesDark lines in the
solar spectrum, discovered by Joseffractional distillation 234f