stable in the atomic nucleus but de-
cays into a proton, an electron, and
an antineutrino with a mean life of
12 minutes outside the nucleus. Its
rest mass is slightly greater than that
of the proton, being 1.674 9286(10) ×
10 –27kg. Neutrons occur in all atomic
nuclei except normal hydrogen. The
neutron wasÜrst reported in 1932 by
the British physicist James Chadwick
(1891–1974).
neutron activation analysis See
activation analysis.
neutron diffraction The scatter-
ing of neutrons by atoms in solids,
liquids, or gases. This process has
given rise to a technique, analogous
to X-ray diffraction techniques,
using aÛux of thermal neutrons
from a nuclear reactor to study solid-
state phenomena. Thermal neutrons
have average kinetic energies of
about 0.025 eV (4 × 10 –21J) giving
them an equivalent wavelength of
about 0.1 nanometre, which is suit-
able for the study of interatomic in-
terference. There are two types of
interaction in the scattering of neu-
trons by atoms: one is the interaction
between the neutrons and the
atomic nucleus, the other is the in-
teraction between the magnetic mo-
ments of the neutrons and the spin
and orbital magnetic moments of the
atoms. The latter interaction has pro-
vided valuable information on anti-
ferromagnetic and ferrimagnetic
materials (see magnetism). Interac-
tion with the atomic nucleus gives
diffraction patterns that complement
those from X-rays. X-rays, which
interact with the extranuclear elec-
trons, are not suitable for investigat-
ing light elements (e.g. hydrogen),
whereas neutrons do give diffraction
patterns from such atoms because
they interact with nuclei.
neutron number Symbol N. The
number of neutrons in an atomic nu-
cleus of a particular nuclide. It is
equal to the difference between the
*nucleon number and the *atomic
number.
Newlands’ lawSee law of oc-
taves.
Newman projection A type of
*projection in which a molecule is
viewed along a bond. See conforma-
tion.newtonSymbol N. The *SI unit of
force, being the force required to
give a mass of one kilogram an accel-
eration of 1 m s–2. It is named after
Sir Isaac Newton (1642–1727).NewtonianÛuid AÛuid in which
the velocity gradient is directly pro-
portional to the shear stress. If two
Ûat plates of area A are separated by
a layer ofÛuid of thickness d and
move relative to each other at a ve-
locity v, then the rate of shear is v/d
and the shear stress is F/A, where F is
the force applied to each. For a New-
tonianÛuid F/A = μv/d, where μis the
constant of proportionality and is
called the Newtonian viscosity. Many
liquids are NewtonianÛuids over a
wide range of temperatures and pres-
sures. However, some are not; these
are called non-NewtonianÛuids. In
suchÛuids there is a departure from
the simple Newtonian relationships.
For example, in some liquids the vis-
cosity increases as the velocity gradi-
ent increases, i.e. the faster the liquid
moves the more viscous it becomes.
Such liquids are said to be dilatant
and the phenomenon they exhibit is
called dilatancy. It occurs in some
pastes and suspensions. More com-
mon, however, is the opposite effect
in which the viscosity depends not
only on the velocity gradient but also
on the time for which it has been
applied. These liquids are said to
exhibit thixotropy. The faster a
thixotropic liquid moves the less vis-369 Newtonian fluid
n