72 



ELECTROMAGNETIC RADIATIONS AND MATTER 



ellipsoidal orbitals first envisioned by Sommerfeld and described by 

 Schroedinger. Each orbital can accommodate two electrons only, according 

 to Wolfgang Pauli's "exclusion principle." The quantitative theory has 

 now been tested experimentally for 36 years, by observation of the "light" 

 emitted by excited atoms, and it describes, with the most beautiful precision 

 known in science today, the observed results (more about this later). The in- 

 ference is that Bohr's guess was right. But nobody knows why! 



Werner Heisenberg's introduction of the "uncertainty principle," and 

 later his new formulation, called wave mechanics, in which all the elementary 

 particles (and hence all matter) are considered to follow the undulations of 

 electromagnetic waves, have only served to strengthen the grasp that this 

 particular atomic model, or theory, has on science. 



The model discloses that there are sublevels in which an electron may find 

 itself within the electron cloud: the s, p, d, and / levels,* or orbitals, as they 

 are called (Figure 4-4). In each of these the electron is confined within a 

 certain spherical or cigar-shaped volume about the nucleus. The orbitals of 

 the outermost electrons of the atom overlap with those of the neighboring 

 atom, and form a "bond." 



p-orbitol 



s-orbito 



schematic 



de Broglie s 

 standing waves 



Figure 4-4. Schematic (exaggerated and distorted) s and p Orbitals with de 

 Broglie's "Pilot Waves," Which are Thought to Guide the Electrons in Their Orbits. 



Working from the inside to the outside, we discuss interatomic binding 

 after a section in which we focus attention on the hard, heavy, positive core 

 of the atom, the nucleus, knowledge about which is so important to the 

 understanding of radioactivity and its biological effects. 



*For sharp, principal, diffuse, and fundamental: descriptive codings used by spectroscopists to 

 describe spectral lines. 



