So that: 1 eV (per molecule or atom) = 96.4152206 kJ/mole. The electron volt is frequently used as an energy unit for single molecules or atoms. In many gas phase experiments (like mass spectrometry), the mole is not a convenient unit. The energetics of chemical reactions are typically expressed in kilojoules per mole. Electron Ionization is the currently accepted term. * Some older literature will refer to EI as electron impact, but this term is not considered accurate. This distribution provides the structural information for interpreting mass spectra and is discussed in detail in the section on interpretation. Changing the ionization energy changes the observed distribution of fragment ions. The abundance of the resulting fragments, often called product ions, is determined by the kinetics of the fragmentation pathways and the ionization energy. Other molecular ions are formed with more energy and undergo fragmentation in the source region. Intact molecular ions are observed from ions produced with little excess energy. For most organic compounds the threshold energy for EI is about 20 eV.īecause a mass spectrum is produced by ionizing many molecules, the spectrum is a distribution of the possible product ions. Other electron voltages may be used to vary the amount of fragmentation produced during ionization. The distribution of energy transferred during ionization and the large number of fragmentation pathways results in a variety of products for a given analyte. Special investigations have shown that e e, the charge on the electron, is equal to E E, the charge on the hydrogen ion hence since e E e E and e/m 1.8 ×107 e / m 1.8 × 10 7, while E/M 104, m M/1800 E / M 10 4, m M / 1800 or the mass of an electron is only 1/1800 1 / 1800 of that of an atom of hydrogen. Since approximately 960 kJ/mole (10 eV) of energy is required to ionize most organic compounds and a typical chemical bond energy is 290 kJ/mole (3 eV), extensive fragmentation is often observed in 70 eV EI mass spectra. There is, however, a distribution of energy and as much as 2800 kJ (30 eV) is transferred to some molecules. h E ( 1 n l o w 2 1 n h i g h 2) 13.6 eV. Bohr explained the hydrogen spectrum in terms of electrons absorbing and emitting photons to change energy levels, where the photon energy is. The potential in Equation 7.4.1 at infinity is chosen to be zero. Bohrs model calculated the following energies for an electron in the shell, n. where k is a constant equal to 9.0 × 109N m2 / C2. In most 70 eV EI experiments, approximately 1400 kJ (15 eV) of energy is transferred during the ionization process. The electric potential V of a point charge is given by. The amount of energy transferred during this process depends up on how fast the electron is traveling and how close it passes to the molecule. ** This is enough energy to cause extensive fragmentation, and at this level small changes in the electron energy do not significantly effect the fragmentation patterns. Since the ionization is produced by a single electron that is accelerated to 70 V, this is commonly referred to as 70 eV EI. If enough energy is transferred by the process, the molecule will eject a valence electron and form a radical cation M The major problem with these statements is the use of 'mass' to mean 'apparent mass' or 'observed mass'. The apparent mass of an electron INCREASES with its speed (relative to the observer). This distortion transfers kinetic energy from the fast-moving electron to the electron cloud of the molecule. The electric charge on an electron is fixed, regardless of its speed. As the electron passes close to the molecule the negative charge of the electron repels and distorts the electron cloud surrounding the molecule. What is the net electric potential V at a space point P from these charges? Each of these charges is a source charge that produces its own electric potential at point P, independent of whatever other changes may be doing. Consider a system consisting of N charges \(q_1,q_2., q_N\). Just as the electric field obeys a superposition principle, so does the electric potential. As noted earlier, this is analogous to taking sea level as \(h = 0\) when considering gravitational potential energy \(U_g = mgh\). It is the potential difference between two points that is of importance, and very often there is a tacit assumption that some reference point, such as Earth or a very distant point, is at zero potential. Ground potential is often taken to be zero (instead of taking the potential at infinity to be zero). The voltages in both of these examples could be measured with a meter that compares the measured potential with ground potential. Hence, any path from a point on the surface to any point in the interior will have an integrand of zero when calculating the change in potential, and thus the potential in the interior of the sphere is identical to that on the surface. Recall that the electric field inside a conductor is zero.
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