electron affinity

Examples of electron affinity in the following topics:

• Electron Affinity

  • Mulliken used a list of electron affinities to develop an electronegativity scale for atoms by finding the average of the electron affinity and ionization potential.
  • A molecule or atom that has a more positive electron affinity value is often called an electron acceptor; one with a less positive electron affinity is called an electron donor.
  • To use electron affinities properly, it is essential to keep track of the sign.
  • Electron affinity follows the trend of electronegativity: fluorine (F) has a higher electron affinity than oxygen (O), and so on.
  • This table shows the electron affinities in kJ/mol for the elements in the periodic table.

• General Trends in Chemical Properties

  • Elements in the same period show trends in atomic radius, ionization energy, electron affinity, and electronegativity.
  • This is because each successive element has an additional proton and electron, which causes the electrons to be drawn closer to the nucleus.
  • Electron affinity also shows a slight trend across a period: metals (the left side of a period) generally have a lower electron affinity than nonmetals (the right side of a period), with the exception of the noble gases which have an electron affinity of zero.
  • The primary determinant of an element's chemical properties is its electron configuration, particularly that of the valence shell electrons.
  • Since the outermost electrons determine chemical properties, those with the same number of valence electrons are generally grouped together.

• Electrolytic Properties

  • Lone electrons cannot usually pass through the electrolyte; instead, a chemical reaction occurs at the cathode that consumes electrons from the anode.
  • Another reaction occurs at the anode, producing electrons that are eventually transferred to the cathode.
  • In batteries for example, two materials with different electron affinities are used as electrodes: outside the battery, electrons flow from one electrode to the other; inside, the circuit is closed by the electrolyte's ions.
  • The mnemonic "LeO said GeR" is useful for remembering "lose an electron in oxidation" and "gain an electron in reduction."
  • The production of this low-energy and stable electron configuration is clearly a favorable process.

• Electronegativity and Oxidation Number

  • Electronegativity is a property that describes the tendency of an atom to attract electrons (or electron density) toward itself.
  • The higher its electronegativity, the more an element attracts electrons.
  • Properties of a free atom include ionization energy and electron affinity.
  • Where electrons are in space is a contributing factor because the more electrons an atom has, the farther from the nucleus the valence electrons will be, and as a result they will experience less positive charge; this is due to their increased distance from the nucleus, and because the other electrons in the lower-energy core orbitals will act to shield the valence electrons from the positively charged nucleus.
  • One way to characterize atoms in a molecule and keep track of electrons is by assigning oxidation numbers.

• Transition Metals

  • Moving horizontally across the periodic table trends in properties such as atomic radius, electronegativity, and electron affinity are observed.
  • They can be mostly attributed to incomplete filling of the electron d-levels:
  • The formation of compounds whose color is due to d–d electronic transitions.
  • An electron jumps from one d-orbital to another.
  • This illustrates the order in which most atoms populate their electron shells.

• Reactive Intermediates

  • Electrophile: An electron deficient atom, ion or molecule that has an affinity for an electron pair, and will bond to a base or nucleophile.
  • Carbenes have only a valence shell sextet of electrons and are therefore electron deficient.
  • In this sense they are electrophiles, but the non-bonding electron pair also gives carbenes nucleophilic character.
  • Carbon radicals have only seven valence electrons, and may be considered electron deficient; however, they do not in general bond to nucleophilic electron pairs, so their chemistry exhibits unique differences from that of conventional electrophiles.
  • Carbanions are pyramidal in shape (tetrahedral if the electron pair is viewed as a substituent), but these species invert rapidly at room temperature, passing through a higher energy planar form in which the electron pair occupies a p-orbital.

• Nucleophilicity

  • Electrophile: An electron deficient atom, ion or molecule that has an affinity for an electron pair, and will bond to a base or nucleophile.
  • Nucleophile: An atom, ion or molecule that has an electron pair that may be donated in forming a covalent bond to an electrophile (or Lewis acid).

• Charge Distribution in Molecules

  • A large local charge separation usually results when a shared electron pair is donated unilaterally.
  • Because of their differing nuclear charges, and as a result of shielding by inner electron shells, the different atoms of the periodic table have different affinities for nearby electrons.
  • The ability of an element to attract or hold onto electrons is called electronegativity.
  • A larger number on this scale signifies a greater affinity for electrons.
  • When two different atoms are bonded covalently, the shared electrons are attracted to the more electronegative atom of the bond, resulting in a shift of electron density toward the more electronegative atom.

• Standard Reduction Potentials

  • Reduction potential (also known as redox potential, oxidation/reduction potential, or Eh) measures the tendency of a chemical species to acquire electrons and thereby be reduced.
  • The more positive the potential, the greater the species' affinity for electrons, or the more the species tends to be reduced.

• Chemical Bonding & Valence

  • Covalent bonding occurs by a sharing of valence electrons, rather than an outright electron transfer.
  • These illustrations use a simple Bohr notation, with valence electrons designated by colored dots.
  • Note that in the first case both hydrogen atoms achieve a helium-like pair of 1s-electrons by sharing.
  • Non-bonding valence electrons are shown as dots.
  • Consequently, these compounds have an affinity for electrons, and they exhibit exceptional reactivity when compared with the compounds shown above.