. Biophysics: concepts and mechanisms. Biophysics. THE STRUCTURE OF MATTER 71 the whole atom. The atomic diameter calculated from Avogadro's number (6 x 1023 atoms per gram atomic weight) and the density of, say, nickel ( 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
. Biophysics: concepts and mechanisms. Biophysics. THE STRUCTURE OF MATTER 71 the whole atom. The atomic diameter calculated from Avogadro's number (6 x 1023 atoms per gram atomic weight) and the density of, say, nickel ( 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 elem
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