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.

• The Periodic Table of Elements

  • A periodic table is a tabular display of elements organized by their atomic numbers, electron configurations, and chemical properties.
  • The elements are organized based on their atomic numbers, electron configurations, and recurring chemical properties.
  • (The terminology of s-, p-, and d- blocks originate from the valence atomic orbitals the element's electrons occupy. ) Some groups have specific names, such as the halogens or the noble gases.
  • Elements in the same period show trends in atomic radius, ionization energy, and electron affinity.
  • This occurs because each successive element has an added proton and electron, which causes the electron to be drawn closer to the nucleus, decreasing the radius.

• 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.

• Crystallographic Analysis

  • Neutron crystallography is often used to help refine structures obtained by x-ray methods or to solve a specific bond; the methods are often viewed as complementary, as x-rays are sensitive to electron positions and scatter most strongly off heavy atoms, while neutrons are sensitive to nucleus positions and scatter strongly off many light isotopes, including hydrogen and deuterium.
  • Electron crystallography has been used to determine some protein structures, most notably membrane proteins and viral capsids.
  • The protocol for completing a successful crystallographic analysis requires production of proteins (cloning, mutagenesis, bacterial culture, etc.), purification of recombinant proteins (such as chromatography of affinity and gel filtration), enzymatic tests and inhibition measurement (spectrophotometry), crystallization, x-rays crystallography and structural analysis, interactions determination (microcalorimetry, fluorescence, BIAcore), conformational analyses (circular dichroism, ultracentrifugation, light scattering), modifications analysis (mass spectrometry), bioinformatics, and molecular modelisation.
  • Distinguish between the three methods of crystallography: X-ray, neturon and electron crystallography