402 



Table 515 {concluded). 

 LANCMUIR ATOM. BOHR ATOM. ATOMIC MAGNETIC FIELD. 



From the emission of nuclear a-particles, afp^e) = PiCj, it seems probable that the nuclei are compounds of He and H 

 nuclei. By the bombardment of the nuclei of atoms up to atomic number 40 with o-parti<;les Rutherford has obtained H 

 but only where H and He nuclei should both occur in the nucleus (Bo, N, Fl, Na, Al, P). Harkins has developed this 

 idea (J. Franklin. Inst. IQ4, 213 et seq., ig22) and shown the much greater frequency in nature of the even-atomic num- 

 bered elements (97.6 per cent in stony meteorites, pg.2 Fe meteorites, 85.6 lithosphere, 5 unknown elements all odd, even 

 radio-active most stable). Elements below atomic number 30 make up 99.99 per cent of all meteorites, 99.85 igneous 

 rocks, 99.95 shale, 99.95 sandstones, 99.85 lithosphere. The stability of the He nucleus may be judged by the energy set 

 free in the formation of He from H. According to "relativity" i g-mass = 9 X lo*" ergs (E = mc'-). The change of 

 mass involved in the formation of i g-atom of He (4.000 g) from 4 g-atoms of Ha (4 X 1.0078 g) = 2.81 X 10" ergs = 

 6.71 X 10" calories, i lb. Hj changed to He equals heat from 10,000 tons coal. The nuclei of light even numbered 

 atoms (most abundant isotope) up to Fe (26) almost wholly of He nuclei. To a ist approximation the a-particle behaves 

 in collision like an elastic oblate spheroid, semi-axes, 8 X 10-'' and 4 X lo-'^ cm. (Lhadwick, Bieler, P. M. 1921). 



The theory of the arrangement of the extra-nuclear electrons has followed two developments; The physicist, to explain 

 radiation phenomena, desires a planetary type of atom (Bohr atom); the chemist, for stereochemical phenomena, one 

 with electrons not in co-planar revolution, rather vibrating in 3-dimensional position (Langmuir atom). 



Langmulr atom (J.Am., Ch. Soc. 41, 868, 1919) postulates a 3-dimensional more or less symmetrical arrangement of the 

 extra-nuclear electrons in concentric shells containing successively, when complete 2, 8, 8, 18, 18, 32 (N = 2(1^ -|- 2^ + 

 2- + 3- + 3- + 4- +)• No outer layer has electrons until the inner have their quota. The electrons in the outer shell 

 determine the valence and the chemical and electrochemical properties of the element. This device very satisfactorily 

 accounts for many chemical structures and reactions. The following table shows the arrangement of the electrons from 

 shell to shell. N gives the number in the inner completed shells; E, the number (valence electrons in the outer shell). 



Bohr Atom: (Phil. Mag. 26, i, 476, 857, 1913; 29, .332, 1915; 30. 394, 1915). The experimental facts and the law 

 of circular electronic orbits limit the electrons to orbits of particular radii. When an electron is disturbed from its 

 orbit, e.g., struck out by a cathode ray, or returns from space to a particular orbit, energy must be radiated. It is sug- 

 gestive that the emission of a /3 ray requires a series of y ray radiations. H does not radiate unless ionized and then 

 gives out a spectrum represented by Balmer's formula v = Nii/ni"- — i/n") where v is the frequency, A^, a constant, 

 and «i for all the lines in the visible spectrum has the value 2, n, the successive integers, 3, 4, s, . . .; if ni = i and n, 

 2, 3, 4 Lyman's ultra-violet series results; if n-i = 3, n, 4, s, 6, . . ., Paschen's infra-red series. These considera- 

 tions led Bohr to his atom and he assumed: (a) a series of circular non-radiating orbdts governed as above; (b) radia- 

 tion taking place only when an electron jumps from one to another of these orbits, the amount radiated and its frequency 

 being determined by hv = /I, — A^, h being Planck's constant and Ax and A., the energies in the two orbits; (c) the 

 various possible circular orbits, for the case of a single electron rotating around a single positive nucleus, to be deter- 

 mined by r = (i/2)t/?k, in which t is a whole number, n is the orbital frequency, and T is the kinetic energy of rotation. 



The remarkable lest of this theory is not its agreement with the H series, which it was constructed to fit, but in the 

 value found for N. From (a), (b), and (c) it follows that N = {2n"e''E-m)/h^ = 3.294 X 10", within i/io per cent of 

 the observed value (Science,_45, p. 327). 



The radii of the stable orbits = T-k-/4n-me*, or the radii bear the ratios 1,4, g, 16, 25. If normal H be assumed to be 

 with its electron in the inmost orbit, then 2a == i.i X lo-S; best determination gives 2.2 X 10-;^. The fact that H 

 emits its characteristic radiations only when ionized favors the theory that the emission process is a settling down to 

 normal condition through a series of possible intermediate states, i. e., a change of orbit is necessary for radiation. That 

 in the stars there are 33 lines in the Balmer series, while in the laboratory we never get more than 12, is easily explica- 

 ble from the Bohr theory, 



Bohr's theory leads to the relationship "j^q — vj^ = vj^ (see X-ray tables), Rydberg-Schuster law. 



For further development, see Sommerfeld, Ann. d. Phys. 51, i, igi6, Paschen, Ann. d. Phys., October, 1916; Harkins, 

 Recent work on the structure of the atom, J. Am. Ch. Soc. 37, p. 1396, 1915; 39, p. 856, 1916. 



Magnetic Held of atom: From the Zeeman effect due to the action of a magnetic field on the radiating electron the 

 strength of the atomic magnetic field comes out about lo^ gauss, 2000 times the most intense field yet obtained by an 

 electromagnet. A similar result is given by the rotation of a number of electrons, Aio^, where A is the atomic weight; 

 for Fe this gives lo^ gauss. For other determinations, see Weiss (J. de Phys. 6, p. 661, 1907; 7, p. 240, 1908), Ritz 

 (Ann. d. Phys. 25, p. 660, I9o8),0xley (change of magnetic susceptibility on crystallization. Phil. Tr. Roy. Soc. 215, 

 p. 95, 1915) and Merritt (fluorescence, 1915); Humphreys, "The Magnetic Field of an Atom, Science, 46, p. 276, 1917. 



Smithsonian Tables. 



