THE STRUCTURE OF MATTER 



71 



the whole atom. The atomic diameter calculated from Avogadro's number 

 (6 x 10 23 atoms per gram atomic weight) and the density of, say, nickel 

 (8.9 g/cc) is found to be approximately 10~ 8 cm (1 A). Therefore the diam- 

 eter of the nucleus is approximately 10 l2 cm. Of primary importance to 

 an understanding of penetration of energetic radiation into tissue was the 

 deduction: the total positive charge is centered at the nucleus, which con- 

 tains also most of the weight of the atom. The negative charge, equal in 

 magnitude to the positive but of negligible weight, is in the orbital electrons. 



Atomic theory then developed rapidly, between 1910 and 1925. Max 

 Planck suggested that light is emitted and absorbed in bundles of energy 

 (quanta); and Niels Bohr postulated that the electrons are held in definite 

 orbits or levels around the nucleus, bound to the nucleus by positive-negative 

 attraction, yet held from each other by negative-negative repulsion, thus pre- 

 serving a definite diameter for the whole atom. 



It was in 1926 that Erwin Schroedinger proposed an expression relating 

 energy to radius, which for the first time gave these qualitative ideas quan- 

 titative expression. It describes a model of the atom in which the electrons 

 exist in a series of levels or orbitals, given the names K, L, M, etc., the 

 K-shell being next to the nucleus. Figure 4-3 illustrates the spherical and 



Figure 4-3. Sommerfeld's Atom with Elliptical 

 (p) and Spherical (s) Orbitals. Three p's are 

 at right angles to one another. Each orbital can 

 hold two electrons, whether both from the one 

 atom or a "shared pair" in a bond. As drawn, 

 this "atom" could accommodate 2 electrons in 

 the K shell (Is) and 8 in the L shell (2-level). 

 Thus it represents atoms from hydrogen (1 elec- 

 tron) up to neon (10 electrons). The 3s, 3p, 

 etc., orbitals, only slightly larger, and not 

 shown, accommodate orbital electrons of 

 elements higher in the periodic table. 



