 Neighbouringgroup Contribution
 Nematic Phase
 Nernst Diffusion Layer
 Nernst Equation
 Nernst Heat Theorem
 Neutron Diffraction
 Newtonian Flow
 Newton’s Law of Cooling
 Node
 Nonbonding Orbital
 Normal Mode
 Normalization
 Nuclear Magnetic Resonance
 Nuclear Overhauser Effect
 Nuclear Statistics
 Nucleation
 Nucleus
N

Published:17 May 2024
Concepts in Physical Chemistry, Royal Society of Chemistry, 2nd edn, 2024, pp. 218227.
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Physical chemistry is the part of chemistry that seeks to account for the properties and transformations of matter in terms of concepts, principles, and laws drawn from physics. This glossary is a compilation of definitions, descriptions, formulae, and illustrations of concepts that are encountered throughout the subject. This section describes the concepts that begin with the letter N; where appropriate, the entries also describe subsidiary but related concepts. Refer to the Directory for a full list of all the concepts treated.
Neighbouringgroup Contribution
Nematic Phase
The nematic phase of a liquid crystal mesophase is the phase in which molecules are aligned parallel to one another but lack other spatial organization (Figure N.1).
Nernst Diffusion Layer
The Nernst diffusion layer (Figure N.2) is a model of the concentration profile at the interface of an electrode and electrolyte solution in which the concentration falls linearly from a plane at a distance δ from the outer Helmholtz plane (OHP).
Nernst Equation
Nernst Heat Theorem
The Nernst heat theorem states that the entropy change accompanying any physical or chemical transformation approaches zero as T → 0. See Third law of thermodynamics.
Neutron Diffraction
Neutrons slowed to speeds close to those characteristic of room temperature have wavelengths close to 100 pm, and hence are suitable for diffraction by molecules. Neutrons pass through the electronic structure and are scattered by interactions with nuclei through the strong force. This scattering can be markedly different for isotopes and for elements that are neighbours in the periodic table. Moreover, a neutron has a magnetic moment, and the interaction of this moment with the electron spin magnetic moment, which is called magnetic scattering, modifies the scattering pattern. Thus, neutron diffraction is especially useful for studying magnetically ordered lattices. In inelastic neutron scattering, the scattering of a neutron results in the transfer of energy between it and the target molecules. An analysis of the spectrum of energy exchange gives information about the dynamical properties of liquid samples.
Newtonian Flow
A fluid is in a state of Newtonian flow if it can be regarded as streaming in a series of layers or laminas (Figure N.3). The layer adjacent to the wall is stationary and successive layers have increasingly greater speeds.
Newton’s Law of Cooling
Node
A node is a point, line, or plane at which a wavefunction passes through zero (Figure N.5). Note that the wavefunction must pass through and not merely approach zero. A radial node is a node at a point in a radial wavefunction; an angular node is a plane dividing regions of opposite sign in an angular wavefunction. A nodal plane occurs in molecular orbital theory where there is complete destructive interference between atomic orbitals contributed by neighbouring atoms. It is typically associated with antibonding character in that region. The wavefunction of the ground state of a system has no nodes; it is generally the case that the energy of a system increases with the number of nodes in its wavefunctions.
Nonbonding Orbital
A nonbonding orbital is a molecular orbital that when occupied is neither bonding nor antibonding between two neighbouring atoms. It is commonly denoted n. Typically, a nonbonding orbital might be a lone pair on one atom or, in an ABC molecule, a molecular orbital that has contributions from A and C but not from B.
Normal Mode
The number of normal modes of a nonlinear molecule consisting of N atoms is 3N – 6; if the molecule is linear then the number of normal modes is 3N – 5. Normal modes cease to be dynamically independent of each other when anharmonicities are present. A normal mode is classified as infrared active if it corresponds to a changing electric dipole moment of the molecule. It is classified as a Raman active if it corresponds to a changing polarizability. The exclusion rule states that a normal mode of a molecule with centre of inversion cannot be both infrared and Raman active (it may be inactive in both).
Normalization
Nuclear Magnetic Resonance
Nuclear magnetic resonance (NMR) is the resonant absorption of radiofrequency electromagnetic radiation by magnetic nuclei in a magnetic field (Figure N.7). In its simplest form, nuclei are exposed to a magnetic field and the frequency of an incident field is varied until a strong resonant absorption is observed.
The resonance of a group of equivalent nuclei is split into a set of absorption frequencies known as fine structure by spin–spin coupling, their magnetic interaction with groups of nearby magnetic nuclei, with a splitting denoted J. If the latter group consists of N nuclei with spin quantum number I = 1/2, the resonance of the first group is spit into N + 1 lines with a binomial intensity distribution (that is, with relative intensities given by Pascal’s triangle).
Modern techniques of NMR are all based on the use of pulses of radiofrequency radiation and the interpretation of the observed signal by using Fourier transform techniques.
Nuclear Overhauser Effect
Nuclear Statistics
The term nuclear statistics refers to the selective occupation of molecular rotational states that stems from the requirement of the Pauli principle. The rotation of a molecule may result in the interchange of identical nuclei, so the wavefunction must change in accord with the principle: that is, be unchanged for bosons (I = 0, 1, 2,…) but change sign for fermions (I = 1/2, 3/2,…). For spin0 bosons, as in CO_{2}, only evenJ states are allowed (Figure N.8).
For pairs of identical spin1/2 fermions, the electronic and rotational contributions to the total wavefunction might all change in phase. Which values of the rotational quantum number, J, are allowed then depends on how the other contributions change (Figure N.9).
The consequences of nuclear statistics include the appearance of rotational absorption spectra and thermodynamic properties, such as heat capacity. As the conversion of nuclear spins from a parallel to an antiparallel relative orientation is typically slow, evenJ and oddJ molecules may constitute separable species. Thus, orthohydrogen (oH_{2}, I_{total} = 1, odd J) and parahydrogen (pH_{2}, I_{total} = 0, even J), with H specifically the fermion ^{1}H, persist and interconvert only slowly. As oddJ rotational states are associated with I_{total} = 1 (with zcomponents +1, 0, and −1) and evenJ rotational states are associated with I_{total} = 0 and its single state, in an equilibrium sample of dihydrogen, specifically diprotium, ^{1}H_{2}, at room temperature, there is about three times as much orthohydrogen as there is parahydrogen.
Nucleation
Nucleation refers to the presence of centres on which droplets of vapour or microcrystals of solid may form to enable a phase transition to occur.
Nucleus
In the nuclear model of an atom, the nucleus is the small, positively charged, massive cluster of nucleons (protons and neutrons) at its centre. A nucleus is characterized by the atomic number, Z, the number of protons present (and therefore the electric charge Ze), and the nucleon number (or mass number), A, the total number of nucleons present. A nuclide is an atom (not just its nucleus) of an element E with a specific atomic number and nucleon number, and is denoted $ E Z A $ . Nuclides with the same value of Z but different values of A (such as $ C 6 12 and C 6 13 $ ) are isotopes of the element E. Nuclides with the same nucleon number (such as $ C 6 12 and B 5 12 $ ) are isobars, and those with the same number of neutrons (such as $ C 6 13 and N 7 14 $ ) are isotones.
Nuclei are also characterized by their spin, their intrinsic angular momentum, with nuclear spin quantum number I. Nuclei with halfinteger values of I are fermions and those with integer values (including 0) are bosons.
Unstable nuclei undergo a variety of transformations, with the ejection of particles (such as αrays or βrays, which consist of $ H 2 4 e 2 + and e \u2212 $ , respectively) or photons (as γradiation) giving rise to radioactivity. They may also capture electrons from the inner extranuclear shells of the atom. If the process results in a change in atomic number, it amounts to the transmutation of the original element.