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What is Electron Affinity – Definition

In chemistry and atomic physics, the electron affinity of an atom or molecule is defined as: the change in energy (in kJ/mole) of a neutral atom or molecule (in the gaseous phase) when an electron is added to the atom to form a negative ion. Electron Affinity

Electron Affinity

In chemistry and atomic physics, the electron affinity of an atom or molecule is defined as:

the change in energy (in kJ/mole) of a neutral atom or molecule (in the gaseous phase) when an electron is added to the atom to form a negative ion.

X + e– → X– + energy        Affinity = – ∆H

In other words, it can be expressed as the neutral atom’s likelihood of gaining an electron. Note that, ionization energies measure the tendency of a neutral atom to resist the loss of electrons. Electron affinities are more difficult to measure than ionization energies.

A fluorine atom in the gas phase, for example, gives off energy when it gains an electron to form a fluoride ion.

F + e– → F–        – ∆H = Affinity = 328 kJ/mol

To use electron affinities properly, it is essential to keep track of sign. When an electron is added to a neutral atom, energy is released. This affinity is known as the first electron affinity and these energies are negative. By convention, the negative sign shows a release of energy. However, more energy is required to add an electron to a negative ion which overwhelms any the release of energy from the electron attachment process. This affinity is known as the second electron affinity and these energies are positive.

Affinities of Non metals vs. Affinities of Metals

  • Metals: Metals like to lose valence electrons to form cations to have a fully stable shell. The electron affinity of metals is lower than that of nonmetals. Mercury most weakly attracts an extra electron.
  • Nonmetals: Generally, nonmetals have more positive electron affinity than metals. Nonmetals like to gain electrons to form anions to have a fully stable electron shell. Chlorine most strongly attracts extra electrons. The electron affinities of the noble gases have not been conclusively measured, so they may or may not have slightly negative values.
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References:
Nuclear and Reactor Physics:
  1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
  2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
  3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
  4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
  5. W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467
  6. G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965
  7. Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
  8. U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
  9. Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.

Advanced Reactor Physics:

  1. K. O. Ott, W. A. Bezella, Introductory Nuclear Reactor Statics, American Nuclear Society, Revised edition (1989), 1989, ISBN: 0-894-48033-2.
  2. K. O. Ott, R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985, ISBN: 0-894-48029-4.
  3. D. L. Hetrick, Dynamics of Nuclear Reactors, American Nuclear Society, 1993, ISBN: 0-894-48453-2.
  4. E. E. Lewis, W. F. Miller, Computational Methods of Neutron Transport, American Nuclear Society, 1993, ISBN: 0-894-48452-4.

See also:

Chemical Properties

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