74 



ELECTROMAGNETIC RADIATIONS AND MATTER 



rapidly into the muon plus a gamma ray. The muon disintegrates into a 

 fast, ionizing electron and two more gamma rays, at sea level. 



The atom and its nucleus were recently detailed in delightful form by 

 Gamov 10 , in a little book highly recommended for its simple, colorful de- 

 scriptions of very complex phenomena. 



Molecular Structure and Binding 



It is the outer, or valence, electrons of the electron cloud which are evi- 

 dently involved in binding atom to atom (Figure 4-4). Two distinct cases, 

 and one intermediate case, have been studied thoroughly. First, the valence 

 electron in "atom 1" can jump into an empty orbital of "atom 2," leaving 

 atom 1 positive and making atom 2 negative. Strong electrostatic binding 

 exists (Coulomb's law) because the charge separation is small. This is the 

 case in all salts, both inorganic and organic. The bond is called ionic. 



Secondly, the electron from atom 1 can simply exchange, or be "shared" 

 with that of atom 2. For instance if each of the two valence electrons is in 

 an s (spherical) orbital, and the orbitals can overlap so that exchange or 

 sharing takes place, a "sigma" bond is formed. If both are in p ("probing") 

 orbitals (cigar-shaped), and if they overlap, a so-called pi (7r) bond is formed 

 (Figure 4-5). Indeed combinations of s and p, called "hybrids," are pos- 

 sible. For example each of the four bonds made by a carbon atom is a hy- 

 brid of one s and three/? valence electrons — imagine, in Figure 4-4, the 2s 

 and 2p electron orbitals as distorted; it is a mixture, called an sp^ hybrid. 

 The four are directed tetrahedrally from each other, like four long noses, 

 each to form a bond (i.e., to share a pair of electrons) with a neighboring 

 atom. In the case of water, each of the p orbitals of oxygen overlaps with s 

 of hydrogen to form a bent (109°) molecule. The bond is called covalent. 



TT-bond 

 electrons 



closed 

 loop 



(b) 



TT bond electrons, open path, 

 \ m obi le 



carbon atoms 



Figure 4-5. Diagrams of Overlapping it Bonds: (a) A closed loop to form a dough- 

 nut of negative charge above the plane of a benzene ring; (b) on a protein with open 

 and ringed molecular structures, in which 7r-bond electrons are somewhat mobile and 

 can transfer charge from one end of the molecule to the other, if forced. 



