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1.8: Magnetic Properties

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    Estimated Time to Read: 5 min

    The magnetic moment of a system measures the strength and the direction of its magnetism. The term itself usually refers to the magnetic dipole moment. Anything that is magnetic, like a bar magnet or a loop of electric current, has a magnetic moment. A magnetic moment is a vector quantity, with a magnitude and a direction. An electron has an electron magnetic dipole moment, generated by the electron's intrinsic spin property, making it an electric charge in motion. There are many different magnetic forms: including paramagnetism, diamagnetism, ferromagnetism, and anti-ferromagnetism. Only paramagnetism and diamagnetism are discussed here.

    Paramagnetism

    Paramagnetism refers to the magnetic state of an atom with one or more unpaired electrons. The unpaired electrons are attracted by a magnetic field due to the electrons' magnetic dipole moments. Hund's Rule states that electrons must occupy every orbital singly before any orbital is doubly occupied. This may leave the atom with many unpaired electrons. Because unpaired electrons can orient in either direction, they exhibit magnetic moments that can align with a magnet. This capability allows paramagnetic atoms to be attracted to magnetic fields. Diatomic oxygen, O2, is paramagnetic; it is attracted to the magnet. In contrast, molecular nitrogen, N2, has no unpaired electrons and is diamagnetic (discussed below); it is therefore unaffected by the magnet.The following video shows liquid oxygen attracted into a magnetic field created by a strong magnet.

    Liquid Oxygen is attracted to a magnetic field: Paramagnetism of Liquid Oxygen, YouTube(opens in new window) [youtu.be]

    Paramagnetism is a form of magnetism whereby materials are attracted to an externally applied magnetic field. Paramagnetic species have one or more unpaired electrons.

    Diamagnetism

    Diamagnetic species are characterized by paired electrons; they have no unpaired electrons. According to the Pauli Exclusion Principle which states that no two identical electrons may take up the same quantum state at the same time, the electron spins are oriented in opposite directions. This causes the magnetic fields of the electrons to cancel out. Thus, there is no net magnetic moment, and the atom cannot be attracted into a magnetic field. In fact, diamagnetic substances are weakly repelled by a magnetic field as demonstrated with the pyrolytic carbon sheet in Figure \(\PageIndex{1}\). Diamagnetism, to some degree, is a property of all materials and always makes a weak contribution to the material's response to a magnetic field. For materials that show other form of magnetism (such paramagnetism), the diamagnetic contribution becomes negligible.

    220px-Diamagnetic_graphite_levitation.jpg
    Figure \(\PageIndex{1}\): Levitating pyrolytic carbon: A small (~6 mm) piece of pyrolytic graphite levitating over a permanent neodymium magnet array (5 mm cubes on a piece of steel). Note that the poles of the magnets are aligned vertically and alternate (two with north facing up, and two with south facing up, diagonally). [Wikipedia]

    Diamagnetic materials are repelled by an applied magnetic field. Diamagnetic species have no unpaired electrons.

    How to determine if a substance is paramagnetic or diamagnetic

    The magnetic form of a substance can be determined by examining its electron configuration. If it includes unpaired electrons, then the substance is paramagnetic. If all electrons are paired, then the substance is diamagnetic. This process can be broken into four steps:

    1. Write the electron configuration
    2. Draw the energy diagram for the valence orbitals by applying Hund's Rule
    3. Look for unpaired electrons
    4. Decide if the substance is paramagnetic (one or more electrons unpaired) or diamagnetic (all electrons paired)
    Example \(\PageIndex{1}\): Chlorine

    Are chlorine atoms paramagnetic or diamagnetic?

    Solution

    1. Write the electron configuration. Cl: 1s2 2s2 2p6 3s2 3p5
    2. Draw the energy diagram for the valence orbitals.
      Slide2.JPG
      Figure \(\PageIndex{2}\): Energy diagram showing the valence orbitals of a Cl atom.
    3. Look for unpaired electrons.
    4. Determine whether the substance is paramagnetic or diamagnetic. Since there is an unpaired electron, the Cl atom is paramagnetic.
    Example \(\PageIndex{2}\): Zinc

    Are zinc atoms paramagnetic or diamagnetic?

    Solution

    1. Write the electron configuration. Zn: 1s2 2s2 2p6 3s2 3p6 4s2 3d10
    2. Draw the energy diagram for the valence orbitals.
      Slide1.JPG
      Figure \(\PageIndex{3}\): Energy diagram showing the valence orbitals of a Zn atom.
    3. Look for unpaired electrons.
    4. Determine whether the substance is paramagnetic or diamagnetic. Since there is are no unpaired electrons, the Zn atom is diamagnetic.
    Caution

    Having an even number of electrons in a subshell does not mean that all electrons in the subshell are paired! According to Hund's rule, no electrons will be paired until all orbitals in a subshell have at least one unpaired electron.

    p2.png
    p2 configuration
    d4.png
    d4 configuration
    d8.png
    d8 configuration
    Figure \(\PageIndex{4}\): Energy diagrams showing diamagnetic electron configurations with even numbers of electrons in the subshell: p2, d4, d8. (CC-BY-NC-SA; Kathryn A. Newton)

    Determining Magnetic Properties from Orbital Diagrams, YouTube(opens in new window) [youtu.be]

    Problems

    Exercise \(\PageIndex{1}\)

    Decide if a boron atom is paramagnetic or diamagnetic.

    Answer

    B: 1s2 2s2 2p1

    This configuration has one unpaired electron in the 2p subshell.Because it has one unpaired electron, it is paramagnetic

    Exercise \(\PageIndex{2}\)

    Decide if a fluoride ion is paramagnetic or diamagnetic.

    Answer

    F: 1s2 2s2 2p5

    F-: 1s2 2s2 2p6

    This configuration has no unpaired electrons. Because it has no unpaired electrons, it is diamagnetic.

    Exercise \(\PageIndex{3}\)

    Decide if a bromine atom is paramagnetic or diamagnetic.

    Answer

    Br: [Ar] 4s2 3d10 4p5

    This configuration has one unpaired electron in the 2p subshell (and four paired electrons). Because it has one unpaired electron, it is paramagnetic.

    Exercise \(\PageIndex{1}\)

    Decide if Fe2+ is paramagnetic or diamagnetic.

    Answer

    Fe: [Ar] 4s2 3d6

    Fe2+: [Ar] 3d6

    This configuration has two paired electrons and four unpaired electrons in the 3d subshell. Because it has at least one unpaired electron, it is paramagnetic.

    References

    1. Pettrucci, Ralph H. General Chemistry: Principles and Modern Applications. 9th. Upper Saddle River: Pearson Prentice Hall, 2007
    2. Sherman, Alan, Sharon J. Sherman, and Leonard Russikoff. Basic Concepts of Chemistry Fifth Edition. Boston, MA: Houghton Mifflin Company, 1992. Print.

    1.8: Magnetic Properties is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Kathryn A. Newton, Northern Michigan University.