The Physical Structure of Atoms

FIGURE 1.8 Thomson s apparatus to measure the electron charge-to-mass ratio, e/rOe. Electrons (cathode rays) stream across the tube from left to right. The electric field alone deflects the beam down, and the magnetic field alone deflects it up. By adjusting the two field strengths, Thomson could achieve a condition of zero net deflection. (( indicates the length of the deflection plates.) 

As soon as the electron flies into the space between the plates, it begins to experience a constant downward force, given by 

The net downward deflection of the electron by the time it escapes from the plates is then 

After the electron escapes from the plates, it experiences no further forces, so it continues in straight-line motion toward the fluorescent screen. This motion carries the electron farther from the undeflected path and magnifies the displacement by the factor 2L/ , where L is the distance from the center of the plates to the screen. When the electron arrives at the screen, the net displacement will be 

All of these quantities could be read off the apparatus except for the velocity of the electron, which was hard to measure directly. 

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Coulomb s law is the conceptual basis of this chapter. All the key equations are devoted to stating this law and using it to describe the physical structure of atoms (their ionization energies, electron affinities, and electronegativities) and the stabilization of chemical bonds. 

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