This radius can also be represented as 20 fm, where 1fm = 10^-15m, the femtometer or fermi unit. We have seen how Rutherford estimated the radius of a silver nucleus to be 2 x 10^-14 m, by observing the angular dependence of alpha-particle scattering. The fact that C-12 nucleus has a mass slightly less than that of its 12 constituent particles indicates that some mass is lost (as energy, E=mc^2) when the particles are combined into a nucleus. The mass of an isolated proton has been measured to be 1.0073u and that of a neutron is 1.0087u (about 0.14% larger). Natural carbon consists mainly of the A=12 isotope (carbon-12) but contains 1.1% of carbon-13, so the atomic weight of carbon is actually 12.011, which represents a weighted average over all the isotopes present.īy definition, the mass of the carbon-12 nucleus is 12u, where u is the atomic mass unit (amu), equal to 1.66 x 10^-27 kg. Hydrogen normally has Z=1, A=1 but can be made with A=2 (deuterium) and A=3 (tritium). Although Z is always the same for a given element, the number of neutrons (and therefore A) is found to vary, giving rise to different isotopes of the element. Together, these two kinds of particles, when present within a nucleus, are called nucleons and their total number A is the mass number of the nucleus, which is closely related to the atomic weight of the corresponding element. We now know that an atomic nucleus contains Z protons (Z being the proton number of the atom, equal to the atomic number of the corresponding element), each of which has a positive charge (+e), and N uncharged neutrons (N being the neutron number). Two years later, Marie Curie named this emission of radiation from atomic nuclei radioactivity, and less 20 years later Rutherford had used this radiation to investigate properties of the atom and its nucleus (including the nuclear size). Our knowledge of nuclear physics started with Becquerel's accidental discovery (1896) that crystals of a uranium salt emit radiation which blackens a photographic emulsion. Since the nucleus is very small (the particles it contains are highly confined), the Heisenberg uncertainty principle tells us that the momentum uncertainly will be large, the energy levels far apart and the energies involved in nuclear reactions correspondingly large (typically MeV). Wave mechanics can be applied to the nucleus of an atom, as well as to its surrounding electrons. The extent of the deflection depends on the mass-to-charge ratio of the ion.Figure references are to the second edition of Modern Physics by Serway, Moses and Moyer (Saunders, 1997) When an electric field is applied, the ions are accelerated into a separate chamber where they are deflected from their initial trajectory by a magnetic field, like the electrons in Thomson’s experiment. First, electrons are removed from or added to atoms or molecules, thus producing charged particles called ions. The technique is conceptually similar to the one Thomson used to determine the mass-to-charge ratio of the electron. Scientists can measure relative atomic masses very accurately, however, using an instrument called a mass spectrometer. Although the difference in mass is small, it is extremely important because it is the source of the huge amounts of energy released in nuclear reactions.īecause atoms are much too small to measure individually and do not have charges, there is no convenient way to accurately measure absolute atomic masses. For example, the ratio of the masses of 1H (hydrogen) and 2H (deuterium) is actually 0.500384, rather than 0.49979 as predicted from the numbers of neutrons and protons present. Once the masses of atoms were determined, the amu could be assigned an actual value:ġ amu = 1.66054 x 10 -24 grams conversely: 1 gram = 6.02214 x 10 23 amuĪlthough the masses of the electron, the proton, and the neutron are known to a high degree of precision ( Table 2.3.1), the mass of any given atom is not simply the sum of the masses of its electrons, protons, and neutrons. Thus, the mass of the hydrogen atom ( 1H) is 1.0080 amu, and the mass of an oxygen atom ( 16O) is 15.995 amu. The atomic mass unit ( amu) was not standardized against hydrogen, but rather, against the 12C isotope of carbon ( amu = 12). As we saw earlier, it is convenient to use a reference unit when dealing with such small numbers: the atomic mass unit. We now know that a hydrogen atom has a mass of 1.6735 x 10 -24 grams, and that the oxygen atom has a mass of 2.6561 X 10 -23 grams. Thus, oxygen was assigned an atomic mass of 16. Hydrogen, the lightest element, was assigned a relative mass of '1', and the other elements were assigned 'atomic masses' relative to this value for hydrogen.
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