ultrasound The use of sound with a frequency higher
than 20,000 Hz. Used to obtain images for medical
diagnostic purposes, especially during pregnancy.
ultraviolet radiation The energy range just beyond
the violet end of the visible spectrum. Although ultravi-
olet radiation constitutes only about 5 percent of the
total energy emitted from the sun, it is the major energy
source for the stratosphere and mesosphere, playing a
dominant role in both energy balance and chemical
composition.
Most ultraviolet radiation is blocked by Earth’s
atmosphere, but some solar ultraviolet radiation pene-
trates the ozone and aids in plant photosynthesis and
the production of vitamin D in humans. Too much
ultraviolet radiation can burn the skin, cause skin can-
cer and cataracts, and damage vegetation.
ultraviolet spectrum (UV) The electromagnetic
spectrum beyond the violet end of the visible spectrum.
Wavelengths in the 400-nm or less range.
umpolung Any process by which the normal alter-
nating donor and acceptor reactivity pattern of a chain,
which is due to the presence of O or N heteroatoms, is
interchanged. Reactivity umpolung is most often
achieved by temporary exchange of heteroatoms (N, O)
by others, such as P, S, and Se.
The original meaning of the term has been
extended to the reversal of any commonly accepted
reactivity pattern. For example, reaction of R–CCX
(X = halide) as a synthon for ′′R–CC+(i.e., ELEC-
TROPHILICacetylene) is an umpolung of the normal,
more common acetylide, R–CC–(i.e., NUCLEOPHILIC)
reactivity.unified atomic mass unit(u or mu) A unit of mass
defined as the mass of one atom of^12 C divided by 12.
Its approximate value is u = (1.660 565±0.000 008 6)
Å E10–27kg.unimolecular SeeMOLECULARITY.unit cell The smallest repeating unit of a crystalline
solid that can be used to describe the entire structure.
Can be used as a template and copied to produce an
entire crystal.unreactive Failing to react with a specified CHEMI-
CAL SPECIES under specified conditions. The term
should not be used in place of STABLE, since a relatively
more stable species may nevertheless be more REACTIVE
than some reference species toward a given reaction
partner.271