Assorted References

• cosmic rays
  • In cosmic ray

    …a high-speed particle—either an atomic nucleus or an electron—that travels through space. Most of these particles come from sources within the Milky Way Galaxy and are known as galactic cosmic rays (GCRs). The rest of the cosmic rays originate either from the Sun or, almost certainly in the case of…

    Read More
• diamagnetism
  • 
    In rock: Basic types of magnetization

    …the orbiting electrons surrounding each atomic nucleus. When an external magnetic field is applied, the orbits are shifted in such a way that the atoms set up their own magnetic field in opposition to the applied field. In other words, the induced diamagnetic field opposes the external field. Diamagnetism is…

    Read More
• gamma rays
  • 
    In electromagnetic radiation: Gamma rays

    During radioactive decay, an unstable nucleus usually emits alpha particles, electrons, gamma rays, and neutrinos spontaneously. In nuclear fission, the unstable nucleus breaks into fragments, which are themselves complex nuclei, along with such particles as neutrons and protons. The resultant stable nuclei or nuclear fragments are usually in a highly…

    Read More
• hydrogen
  • 
    In hydrogen: Ortho-hydrogen and para-hydrogen

    …the magnetic properties of the atoms. Normally, transformations of one type into the other (i.e., conversions between ortho and para molecules) do not occur and ortho-hydrogen and para-hydrogen can be regarded as two distinct modifications of hydrogen. The two forms may, however, interconvert under certain conditions. Equilibrium between the two…

    Read More
• isotopes
  • 
    In spectroscopy: Perturbations of levels

    …the finite mass of the nucleus. For heavier atoms, the main contribution comes from the fact that the volume of the nucleus increases as the number of neutrons increases. The nucleus may behave as a small magnet because of internal circulating currents; the magnetic fields produced in this way may…

    Read More
  • 
    In isotope: Physical properties associated with isotopes

    …the interplay, distinctive for each nucleus, of nuclear and electrostatic forces between neutrons, protons, and electrons. Helium-6, for example, is radioactive, whereas helium-4 is stable. Second, the spatial distribution of the protons in the nucleus affects in measurable ways the behaviour of the surrounding electrons. The addition of one neutron…

    Read More
• magic numbers
  • In magic number

    …gases exactly correspond to the atomic magic numbers.

    Read More
• magnetic fields
  • 
    In magnetism: Repulsion or attraction between two magnetic dipoles

    …in a magnetic field, their nuclei (protons) tend to have their magnetic moments preferentially aligned in the direction of the field. The magnetic potential energy of the nuclei is calculated according to equation (7) as −mB. Inverting the direction of the dipole moment requires an energy of 2mB, since the…

    Read More
• molecules
  • 
    In spectroscopy: General principles

    …a collection of positively charged atomic nuclei surrounded by a cloud of negatively charged electrons. Its stability results from a balance among the attractive and repulsive forces of the nuclei and electrons. A molecule is characterized by the total energy resulting from these interacting forces. As is the case with…

    Read More
• nuclear fission
  • 
    In nuclear fission: Structure and stability of nuclear matter

    Nuclei consist of nucleons (neutrons and protons), the total number of which is equal to the mass number of the nucleus. The actual mass of a nucleus is always less than the sum of the masses of the free neutrons and protons that constitute it,…

    Read More
• particle accelerators
  • 
    In particle accelerator: History

    …research into the properties of atomic nuclei and subatomic particles. Starting with British physicist Ernest Rutherford’s discovery in 1919 of a reaction between a nitrogen nucleus and an alpha particle, all research in nuclear physics until 1932 was performed with alpha particles released by the decay of naturally

    Read More
• periodic table structure
  • In periodic table

    …positive electrical charges of the atomic nuclei expressed in electronic units. In subsequent years great progress was made in explaining the periodic law in terms of the electronic structure of atoms and molecules. This clarification has increased the value of the law, which is used as much today as it…

    Read More
• resonance-ionization mass spectrometry
  • 
    In spectroscopy: Resonance-ionization mass spectrometry

    …determining the relative weights of atomic nuclei, the mass spectrometer is one of the most useful instruments used by analytical chemists. If two atoms with the same number of protons (denoted Z) contain different numbers of neutrons, N, they are referred to as isotopes; if they have the same

    Read More
• subatomic particles
  • 
    In atom: Nuclear energy

    …can be derived from the atomic nucleus. The basic physical principle behind this fact is that the total mass present after a nuclear reaction is less than before the reaction. This difference in mass, via the equation E = mc², is converted into what is called nuclear energy.

    Read More
  • 
    In subatomic particle

    …the small but very dense nucleus of the atom, the positively charged protons and the electrically neutral neutrons. But these basic atomic components are by no means the only known subatomic particles. Protons and neutrons, for instance, are themselves made up of elementary particles called quarks, and the electron is

    Read More
• transuranium elements
  • 
    In transuranium element: Nuclear structure and stability

    …nuclear reactions and nuclear and atomic structure. Study of the known transuranium elements also helps in predicting the properties of yet-undiscovered isotopes and elements as a guide to the researcher who can then design experiments to prepare and identify them. As shown in the figure, the known isotopes can be…

    Read More

atomic structure

• 
  In atom: The nucleus

  The primary constituents of the nucleus are the proton and the neutron, which have approximately equal mass and are much more massive than the electron. For reference, the accepted mass of the proton is 1.672621777 × 10⁻²⁴ gram, while that of the…

  Read More
• 
  In spectroscopy: Basic atomic structure

  …charged electrons bound to a nucleus containing an equal number of positively charged protons. The nucleus contains a certain number (Z) of protons and a generally different number (N) of neutrons. The diameter of a nucleus depends on the number of protons and neutrons and is typically 10⁻¹⁴ to 10⁻¹⁵

  Read More
• 
  In chemical element: The structure of atoms

  …within an atom, a small nucleus, which generally contains both protons and neutrons, is surrounded by a swarm, or cloud, of electrons. The fundamental properties of these subatomic particles are their weight and electrical charge. Whereas protons carry a positive charge and electrons a negative one, neutrons are electrically neutral.…

  Read More

• magic number
  • In magic number

    The magic numbers for nuclei are 2, 8, 20, 28, 50, 82, and 126. Thus, tin (atomic number 50), with 50 protons in its nucleus, has 10 stable isotopes, whereas indium (atomic number 49) and antimony (atomic number 51) have only 2 stable isotopes apiece. The doubly magic alpha…

    Read More

energy states

• 
  In spectroscopy: Radio-frequency spectroscopy

  the electrons and the nucleus and by the mutual repulsion of the electrons. Electrons and nuclei have magnetic properties in addition to these electrostatic properties. The spin-orbit interaction has been discussed above (see Fine and hyperfine structure of spectra). Other, usually weaker, magnetic interactions within the atom exist between…

  Read More

• excitation
  • In excitation

    …to a system—such as an atomic nucleus, an atom, or a molecule—that results in its alteration, ordinarily from the condition of lowest energy (ground state) to one of higher energy (excited state).

    Read More

nuclear spin

• In spin

  …with a subatomic particle or nucleus and measured in multiples of a unit called the Dirac h, or h-bar (ℏ), equal to the Planck constant divided by 2π. For electrons, neutrons, and protons, the multiple is 0.5; pions have zero spin. The total angular momentum of nuclei more complex than…

  Read More

• angular momentum
  • 
    In quantum mechanics: Cesium clock

    Like electrons, many atomic nuclei have spin. The spin of these nuclei produces a set of small effects in the spectra, known as hyperfine structure. (The effects are small because, though the angular momentum of a spinning nucleus is of the same magnitude as that of an electron,…

    Read More
• chemical analysis
  • 
    In chemical analysis: Nuclear magnetic resonance

    …sufficient to cause a spinning nucleus in some atoms to move to a different spin state in the presence of a magnetic field. Consequently, nuclear magnetic resonance spectrometry is useful for examining atomic nuclei and the transitions between their possible spin states. Because nuclei from different atoms have different possible…

    Read More
• resonance line
  • 
    In magnetic resonance: Nuclear magnetic resonance

    …increased by the interaction between nuclear spins. As already mentioned in connection with motion narrowing in liquids, the usual magnetic dipolar interactions are averaged out by molecular motion and do not split the NMR spectra. There exists, however, an indirect interaction between nuclear spins, caused by the electrons, that splits…

    Read More

work of

• Bohr
  • In Aage N. Bohr

    …the asymmetrical shapes of certain atomic nuclei.

    Read More
• Mottelson
  • In Ben Roy Mottelson

    …distort the shape of the nucleus, thus challenging the widely accepted theory that all nuclei are perfectly spherical. Subsequently it was discovered that such asymmetries occur in atoms of all elements.

    Read More
• Rainwater
  • In James Rainwater

    …the asymmetrical shapes of certain atomic nuclei.

    Read More
• Rutherford
  • 
    In Rutherford model

    …positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun.

    Read More
  • 
    In principles of physical science: Development of the atomic theory

    …determined that a small, massive nucleus carries all the positive charge whose magnitude, expressed as a multiple of the fundamental charge of the proton, is the atomic number. An equal number of electrons carrying a negative charge numerically equal to that of the proton form a cloud whose diameter is…

    Read More
  • 
    In Ernest Rutherford: University of Cambridge

    … is the same as the nucleus of an ordinary helium atom—consisting of two protons and two neutrons—and the beta particle is the same as an electron or its positive version, a positron.) For the next several years these radiations were of primary interest; later the

    Read More