nucleosynthesis The synthesis of
chemical elements by nuclear
processes. There are several ways in
which nucleosynthesis can take
place. Primordial nucleosynthesis
took place very soon after the big
bang, when the universe was ex-
tremely hot. This process was respon-
sible for the cosmic abundances
observed for light elements, such as
helium. Explosive nucleosynthesis
can also occur during the explosion
of a supernova. However, stellar nu-
cleosynthesis, which takes place in
the centre of stars at very high tem-
peratures, is now the principal form
of nucleosynthesis. The exact process
occurring in stellar nucleosynthesis
depends on the temperature, density,
and chemical composition of the
star. The synthesis of helium from
protons and of carbon from helium
can both occur in stellar nucleosyn-
thesis.
nucleotideAn organic compound
consisting of a nitrogen-containing
purine or pyrimidine base linked
to a sugar (ribose or deoxyribose) and
a phosphate group. DNA and RNA
are made up of long chains of nu-
cleotides (i.e. polynucleotides). Com-
pare nucleoside.
nucleusThe central core of an
atom that contains most of its mass.
It is positively charged and consists
of one or more nucleons (protons or
neutrons). The positive charge of the
nucleus is determined by the number
of protons it contains (see atomic
number) and in the neutral atom this
is balanced by an equal number of
electrons, which move around the
nucleus. The simplest nucleus is the
hydrogen nucleus, consisting of one
proton only. All other nuclei also
contain one or more neutrons. The
neutrons contribute to the atomic
mass (see nucleon number) but not
to the nuclear charge. The most mas-
sive nucleus that occurs in nature is
uranium–238, containing 92 protons
and 146 neutrons. The symbol used
for this *nuclide is^23892 U, the upper
Ügure being the nucleon number and
the lowerÜgure the atomic number.
In all nuclei the nucleon number (A)
is equal to the sum of the atomic
number (Z) and the neutron number
(N), i.e. A = Z + N.nuclideA type of atom as charac-
terized by its *atomic number and its
*neutron number. An *isotope refers
to a series of different atoms that
have the same atomic number but
different neutron numbers (e.g. ura-
nium–238 and uranium–235 are iso-
topes of uranium), whereas a nuclide
refers only to a particular nuclear
species (e.g. the nuclides ura-
nium–235 and plutonium–239 areÜs-
sile). The term is also used for the
type of nucleus.
nylonAny of various synthetic
polyamideÜbres having a protein-
like structure formed by the conden-
sation between an amino group of
one molecule and a carboxylic acid
group of another. There are three
main nylonÜbres, nylon 6, nylon 6,6,
and nylon 6,10. Nylon 6, for example
Enkalon and Celon, is formed by the
self-condensation of 6-aminohexa-
noic acid. Nylon 6,6, for example Bri
nylon, is made by polycondensation
of hexanedioic acid (adipic acid) and
1,6-diaminohexane (hexamethylene-
diamine) having an average formula
weight between 12 000 and 15 000.
Nylon 6,10 is prepared by polymeriz-
ing decanedioic acid and 1,6-diamino-
hexane.381 nylon
n