Debye

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This article is about the Debye unit. For the physicist, see Peter Debye.

The debye (symbol: D) is a CGS unit[1] (a non-SI metric unit) of electric dipole moment[note 1] named in honor of the physicist Peter J. W. Debye. It is defined as 1 × 10−18 statcoulomb-centimeter.[note 2] Historically the debye was defined as the dipole moment resulting from two charges of opposite sign but an equal magnitude of 10-10 statcoulomb[note 3] (generally called e.s.u. (electrostatic unit) in older literature), which were separated by 1 ångström.[note 4] This gave a convenient unit for molecular dipole moments.

1 D  = 10-18 statC·cm
= 10-10 esu·Å[note 2]
= 1⁄299,792,458 × 10−21 C·m[note 5]
≈ 3.33564 × 10−30 C·m
≈ 1.10048498 × 1023 qPlP
≈ 0.393430307 ea0[2]

Typical dipole moments for simple diatomic molecules are in the range of 0 to 11 D. Symmetric homoatomic species, e.g. chlorine, Cl2, have zero dipole moment and highly ionic molecular species have a very large dipole moment, e.g. gas phase potassium bromide, KBr, with a dipole moment of 10.5 D.[3]

The debye is still used in atomic physics and chemistry because SI units are inconveniently large. The smallest SI unit of electric dipole moment is the yoctocoulomb-meter, which is roughly 300,000 D.[note 6] There is currently no satisfactory solution to this problem of notation without resorting to the use of scientific notation.

Notes

  1. ^ Electric dipole moment is defined as charge times displacement:
    p = qr
  2. ^ a b The statcoulomb is also known as the franklin or electrostatic unit of charge.
    1 statC  =1 Fr
    =1 esu
  3. ^ 10-10 statcoulomb is approximately 0.2083 units of elementary charge.
  4. ^ The ångström is of an order of magnitude close to that of a typical covalent bond.
    1 Å  = 100 pm
    = 10-8 cm
    = 10-10 m
  5. ^ One debye equals 1 × 10−21 C·m2/s divided by the speed of light in vacuum. Conversely 1 C·m = 2.99792458 × 1029 D.
  6. ^ Yocto-, with a value of 10−24, is the smallest SI prefix. Note that SI disallows the application of prefixes to both members of a compound unit or the compounding of prefixes, thus ruling out such units as the femtocoulomb-femtometer or the microyoctocoulomb-meter (both approximately 0.3 D) respectively.

References

  1. ^ CGS units R. Rowlett (University of North Carolina at Chapel Hill)
  2. ^ Atomic unit of electric dipole moment NIST
  3. ^ Physical chemistry 2d Edition (1966) G.M. Barrow McGraw Hill
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