The Electron

Submitted by jwmoore on Wed, 01/12/2011 - 01:28

Near the middle of the nineteenth century the English chemist and physicist Michael FaradayThe electric charge carried by one mole of electrons, 9.648 670 x 104 C mol-1; abbreviated F. (1791 to 1867) established a connection between electricity and chemical reactions. He already knew that an electric current flowing into certain molten compounds through metalAn element characterized by a glossy surface, high thermal and electrical conductivity, malleability, and ductility. plates called electrodes could cause reactions to occur. Samples of different elements would deposit on the electrodes. Faraday found that the same quantity of electric charge was required to produce 1 mol of any element whose valence was 1. Twice that quantity of charge would deposit 1 mol of an element whose valence was 2, and so on. Electric charge is measured in units called coulombs, abbreviated C. One coulomb is the quantity of charge which corresponds to a current of one ampere flowing for one second. It was found that 96500 C of charge was required to deposit on an electrode l mol of an element whose valence is l. Faraday’s experiments strongly suggested that electricity, like matterAnything that occupies space and has mass; contrasted with energy., consists of very small indivisible particles. The name 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. was given to these particles, and an electric current came to be thought of as a flow of electrons from one place to another. When such a current flows into a chemical compoundA substance made up of two or more elements and having those elements present in definite proportions; a compound can be decomposed into two or more different substances., one electron is required for each 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. of a univalent element deposited on an electrode, two electrons for each atom of an element whose valence is 2, and so on. Thus an electric charge of 96500 C corresponds to 1 mol of indivisible electric particles (electrons). The relationship between electricity and atomic structure was further clarified by experiments involving cathodeThe electrode in an electrochemical cell where reduction occurs; the negatively charged electrode in a vacuum tube.-ray tubes in the 1890s. A cathode-ray tube can be made by pumping most of the air or other gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container. out of a glassA solid material that does not have the long-range order of a crystal lattice; an amorphous solid. A glass melts over a range of temperatures instead of having the definite melting temperature characteristic of crystalline solids. tube and applying a high voltage to two metal electrodes inside. If ZnS or some other phosphor is placed on the glass at the end of the tube opposite the negatively charged electrode (cathode), the ZnS emits light. This indicates that some kind of rays are streaming away from the cathode. When passed between the poles of a magnet, these cathode raysElectrons emitted from the surface of a cathode in a vacuum tube. behave the same way as the β particles described earlier. The fact that they were very small electrically charged particles led the English physicist J. J. Thomson (1856 to 1940) to identify them with the electrons of Faraday’s experiments. Thus cathode rays are a beam of electrons which come out of the solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. metal of the cathode. They behave exactly the same way no matter what the electrode is made of or what gas is in the tube. These observations allow one to conclude that electrons must be constituents of all matter. In addition to being deflected by a magnet, the electron beam in a cathode-ray tube can be attracted toward a positively charged metal plate or repelled from a negative plate. By adjusting such electrodes to exactly cancel the deflection produced by a magnet of known strength, Thomson was able to determine that the ratio of charge to massA measure of the force required to impart unit acceleration to an object; mass is proportional to chemical amount, which represents the quantity of matter in an object. for an electron is 1.76 × 108 C/g. This is a rather large ratio. Either each electron has a very large charge, or each has a very small mass. We can see which by using Faraday’s result that there are 96 500 C mol–1 of electrons

\frac{\text{96 500 C mol}^{-\text{1}}}{\text{1}\text{.76 }\times \text{ 10}^{\text{8}}\text{ C g}^{-\text{1}}}=\text{5}\text{.48 }\times \text{ 10}^{-\text{4}}\text{ g mol}^{-\text{1}}

Thus the molar massThe mass of a mole of substance; the same as molecular weight for molecular substances. of an electron is 5.48 × 10–4 g mol–1, and if we think of the electron as an “atom“(or indivisible particle) of electricity, its atomic weightThe average mass of the naturally occurring isotopes of an element, taking into account the different natural abundances of the isotopes. Expressed relative to the value of exactly 12 for carbon-12; also called atomic mass. would be 0.000548—only \tfrac{1}{1837} that of hydrogen, the lightest element known. In 1909 the American physicist Robert A. Millikan (1863 to 1953) was able to determine the charge on an electron independently of its mass. His value of 1.6 × 10–19 C can be combined with Thomson’s charge-to-mass ratio to give an independent check on the molar mass for the electron

\frac{\text{1}\text{.60 }\times \text{ 10}^{-\text{19}}\text{ C}}{\text{1}\text{.76 }\times \text{ 10}^{\text{8}}\text{ C g}^{-\text{1}}}\text{ }\times \text{ 6}\text{.022 }\times \text{ 10}^{\text{23}}\text{ mol}^{-\text{1}}=\text{5}\text{.47 }\times \text{ 10}^{-\text{4}}\text{ g mol}^{-\text{1}}

thus confirming that the electron has much less mass than the lightest atom. (The quantity 1.6 × 10–19 C is often represented by the symbol e. Thus the charge on a single electron is –e = –1.6 × 10–19 C. The minus sign indicates that the electron is a negatively charged particle.)