DIPOLE, DIPOLE-INDUCED DIPOLE, and LONDON FORCES
(instantaneous induced dipole-induced dipole forces).
The term is sometimes used loosely for the totality
of nonspecific attractive or repulsive intermolecular
forces.
van der Waals radius Radius of an imaginary hard
sphere used to model the atom, determined from mea-
surements of atomic spacing between pairs of
unbonded atoms in crystals. Named for Johannes
Diderik van der Waals, winner of the 1910 Nobel Prize
in physics.
vapor A gas created by boiling or evaporating a
liquid.
vapor pressure Pressure of a vapor at the surface of
its parent liquid.
ventilation Ventilation is the passage of air into and
out of the respiratory tract. Ventilation exchange (VE)
is the exchange of oxygen and carbon dioxide and
other gases during the passage of air into and out of
the respiratory passages.
visible light The portion of the electromagnetic spec-
trum that we perceive. This excludes radio waves,
microwaves, infrared light, ultraviolet light, X rays,
and gamma rays. Each of the visible light wavelengths
are detected as various colors (red, orange, yellow,
green, blue, indigo, and violet, along with various com-
binations and shades of these colors) by the human eye
based on their wavelength, ranging in wavelength from
about 400 nm to about 700 nm. It travels at the same
speed as all other radiation (186,000 miles per second),
and its wavelength is longer than ultraviolet light but
shorter than X rays. Violet has the shortest wavelength,
while red has the longest.
See alsoENERGY.
vitamin An essential organic nutrient that is needed
in small amounts by an organism for metabolism and
other processes. Organisms either synthesize them or
obtain them in other ways. Examples of vitamins are
vitamin C and vitamin E, both antioxidants. A vitamin
usually functions as a coenzyme or a component of a
coenzyme and is soluble in either water or organic sol-
vent. The lack of certain vitamins can lead to disease
such as in rickets (vitamin D), tooth decay (vitamin K),
bone softening (vitamin D), or night blindness (vitamin
A). Other vitamins include vitamin B1 (thiamin), vita-
min B2 (riboflavin), niacinamide (niacin-vitamin B3),
vitamin B6 (pyridoxine), vitamin B12 (COBALAMIN),
pantothenic acid (vitamin B5), pyridoxal (vitamin B6),
phylloquinone (vitamin K), biotin, folic acid, inositol,
choline, and PABA (para amino benzoic acid). Vitamin
supplements are a billion-dollar-a-year industry.vitamin B12 SeeCOBALAMIN.volt The electromotive force that causes current to
flow. One volt equals one joule of energy per coloumb
of charge.voltage Potential difference between two electrodes.276 van der Waals radius
Scanning electron micrograph of crystals of vitamin C (ascorbic
acid) showing monoclinic crystal structure (characterized by
three unequal axes, one pair of which are not at right angles).
Magnification ×140.(Courtesy of Dr. Jeremy Burgess/Science
Photo Library)