nucleus

(noun)

the massive, positively charged central part of an atom, made up of protons and neutrons

Related Terms

Examples of nucleus in the following topics:

  • Photon Interactions and Pair Production

    • Pair production refers to the creation of an elementary particle and its antiparticle, usually when a photon interacts with a nucleus.
    • Below is an illustration of pair production, which refers to the creation of an elementary particle and its antiparticle, usually when a photon interacts with a nucleus.
    • In nuclear physics, this reaction occurs when a high-energy photon (gamma rays) interacts with a nucleus.
    • The nucleus in the process carries away (or provides) access momentum.
    • Describe process of pair production as the result of photon interaction with nucleus

  • Multielectron Atoms

    • In hydrogen-like atoms (those with only one electron), the net force on the electron is just as large as the electric attraction from the nucleus.
    • However, when more electrons are involved, each electron (in the $n$-shell) feels not only the electromagnetic attraction from the positive nucleus, but also repulsion forces from other electrons in shells from '1' to '$n$'.
    • Therefore, these electrons are not as strongly bonded to the nucleus as electrons closer to the nucleus.
    • As an approximation, the effective nuclear charge on each electron can be estimated by: Zeff=Z−σZ_\text{eff} = Z - \sigma, where $Z$ is the number of protons in the nucleus and σ\sigma is the average number of electrons between the nucleus and the electron in question. σ\sigma can be found by using quantum chemistry and the Schrodinger equation or by using Slater's empirical formula.
    • A multielectron atom with inner electrons shielding outside electrons from the positively charged nucleus

  • Nuclear Size and Density

    • However, the nucleus can be modelled as a sphere of positive charge for the interpretation of electron scattering experiments: because there is no definite boundary to the nucleus, the electrons "see" a range of cross-sections, for which a mean can be taken.
    • Rutherford was able to put an upper limit on the radius of the gold nucleus of 34 femtometers (fm).
    • This gives a charge radius for the gold nucleus ($A=197$) of about 7.5 fm.
    • The nuclear density for a typical nucleus can be approximately calculated from the size of the nucleus:
    • Note that the image is not to scale; in reality the nucleus is vastly smaller than the electron shell.

  • Quantum-Mechanical View of Atoms

    • Atom is a basic unit of matter that consists of a nucleus surrounded by negatively charged electron cloud, commonly called atomic orbitals.
    • The atom is a basic unit of matter that consists of a nucleus surrounded by negatively charged electrons.
    • The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons.
    • The electrons of an atom are bound to the nucleus by the electromagnetic (Coulomb) force.
    • This is an illustration of the helium atom, depicting the nucleus (pink) and the electron cloud distribution (black).

  • Binding Energy and Nuclear Forces

    • To disassemble a nucleus into unbound protons and neutrons would require working against the nuclear force.
    • Conversely, energy is released when a nucleus is created from free nucleons or other nuclei—known as the nuclear binding energy.
    • Because of mass-energy equivalence (i.e., Einstein's famous formula $E=mc^2$), releasing this energy causes the mass of the nucleus to be lower than the total mass of the individual nucleons (leading to "mass deficit").
    • Nevertheless, they are strong enough to bind neutrons and protons over short distances, as well as overcome the electrical repulsion between protons in the nucleus.
    • A model of the atomic nucleus showing it as a compact bundle of the two types of nucleons: protons (red) and neutrons (blue).

  • Beta Decay

    • Beta decay is a type of radioactive decay in which a beta particle (an electron or a positron) is emitted from an atomic nucleus.
    • Beta decay is a type of radioactive decay in which a beta particle (an electron or a positron) is emitted from an atomic nucleus, as shown in .
    • Beta decay does not change the number of nucleons, A, in the nucleus; it changes only its charge, Z.
    • A beta-stable nucleus may undergo other kinds of radioactive decay (for example, alpha decay).
    • β decay in an atomic nucleus (the accompanying antineutrino is omitted).

  • Alpha Decay

    • In alpha decay an atomic nucleus emits an alpha particle and transforms into an atom with smaller mass (by four) and atomic number (by two).
    • Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle that consists of two protons and two neutrons, as shown in .
    • Because an alpha particle is the same as a helium-4 nucleus, which has mass number 4 and atomic number 2, this can also be written as:
    • Alpha decay is the most common cluster decay because of the combined extremely high binding energy and relatively small mass of the helium-4 product nucleus (the alpha particle).
    • An atomic nucleus emits an alpha particle and thereby transforms ("decays") into an atom with a mass number smaller by four and an atomic number smaller by two.

  • Nuclear Stability

    • These quantum shells correspond to energy levels within the shell model of the nucleus.
    • An atom with an unstable nucleus, called a radionuclide, is characterized by excess energy available either for a newly created radiation particle within the nucleus or via internal conversion.
    • An atomic nucleus emits an alpha particle and thereby transforms ("decays") into an atom with a mass number smaller by four and an atomic number smaller by two.

  • The Bohr Model of the Atom

    • The laws of classical mechanics predict that the electron should release electromagnetic radiation while orbiting a nucleus (according to Maxwell's equations, accelerating charge should emit electromagnetic radiation).
    • Because the electron would lose energy, it would gradually spiral inwards, collapsing into the nucleus.
    • The electrons can only orbit stably, without radiating, in certain orbits (called by Bohr the "stationary orbits"): at a certain discrete set of distances from the nucleus.
    • The significance of the Bohr model is that the laws of classical mechanics apply to the motion of the electron about the nucleus only when restricted by a quantum rule.

  • Conservation of Nucleon Number and Other Laws

    • In gamma decay, an excited nucleus releases gamma rays, but its proton (Z) and neutron (A-Z) count remain the same:
    • In beta decay, a nucleus releases energy and either an electron or a positron.
    • Electron capture has the same effect on the number of protons and neutrons in a nucleus as positron emission.
    • However, rather than being destroyed, the two protons and two neutrons an atom loses in alpha decay are released as a helium nucleus.
    • Consider, for example, the multistep reaction that occurs when a U-235 nucleus accepts a neutron, as in :