Electron Affinities

Submitted by ChemPRIME Staff on Thu, 12/09/2010 - 17:57

ElectronA negatively charged, sub-atomic particle with charge of 1.602 x 10-19 coulombs and mass of9.109 x 1023 kilograms; electrons have both wave and particle properties; electrons occupy most of the volume of an atom but represent only a tiny fraction of an atom's mass. affinities are more difficult to measure experimentally than are ionizationA process in which an atom, molecule, or negative ion loses an electron; a process in which a covalent molecule reacts with a solvent to form positive and negative ions; for example, a weak acid reacting with water to form its conjugate base (an anion) and a hydrogen (hydronium) ion. energies, and far fewer values are available. The relationship of the periodic tableA chart showing the symbols of the elements arranged in order by atomic number and having chemically related elements appearing in columns. with those electron affinities that have been measured or estimated from calculations can be seen on the table of ionization energies and electron affinities, seen below.

Image:Ionization Energies and Electron Affinities.jpg

* Electron affinities marked with an asterisk (*) have been obtained from theoretical calculations rather than experimental measurements. The heavy colored line separates metals (ionization energyThe quantity of energy required to remove an electron from a neutral atom or molecule or from a positive ion. usually below about 800 kJ mol–1) from nonmetals.

It is not easy to discern many obvious regularities in this table, especially since some of the electron-affinity values quoted are negative, indicating that energy is sometimes required to force an extra electron onto an atomThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume.. Nevertheless, it is quite obvious which of the periodic groups correspond to the highest electron affinities. All the halogens have values of about 300 kJ mol–1 while the group VI nonmetals have somewhat lower values, in the region of 200 kJ mol–1 or less. The high electron affinities of the halogens are a result of their having an almost complete outer shell of electrons. The elementA substance containing only one kind of atom and that therefore cannot be broken down into component substances by chemical means. fluorine, for example, has the structure 1s22s22p5, in which one of the 2p orbitals contains but one electron. If an extra electron is added to this atom to form a fluoride ion, the electron can pair with the electron in the half-filled 2p orbital. The added electron will be shielded from the nucleusThe collection of protons and neutrons at the center of an atom that contains nearly all of the atoms's mass. by the 1s electrons, but the 2s and 2p electrons are in the same shell and will shield it rather poorly. There will thus be quite a large effective nuclear chargeeffective nuclear chargeIn a multi-electron atom, the positive charge that a particular electron experiences; depends on the positive charge on the nucleus, the negative charges on all other electrons, and the locations of those electrons relative to the electron in question. (a rough estimate is +5) attracting the added electron. Because of this overall attraction, energy will be released when the electron is captured by the fluorine atom. Similar reasoning also explains why oxygen also has a high electron affinityThe energy change that occurs as an atom or negative ion accepts an electron. The first electron affinity applies to a neutral atom combining with an electron; the second electron affinity applies to a minus-one ion accepting an electron; etc. Sometimes defined as negative when the negative ion is more stable than the neutral atom and sometimes defined as positive for the same circumstance; check the definition in any source of data.. Here, though, the nuclear charge is smaller, and the attraction for the added electron distinctly less.