TABLE 515. 



4OI 



ELECTRONS. RUTHERFORD ATOM. BOHR ATOM. MAGNETIC FIELD OF ATOM- 



References: Millikan, The Electron, 1917; Science, 45, 421, 1917; Humphreys. Science. 46, 273, 1917; Lodge 

 Nature, 104, 15 and 82. 1919; Thomson, Conduction of Electricity through Gases; Campbell. Modern Electrical 

 Theory; Lorentz, The Theory of Electrons; Richardson, The Electron Theory of Matter, 1914. 



Electron: an elementary + or unit of electricity. 



Free negative electron: (corpuscle, J. J. Thomson); mass = 9.01 X io-g - 1/1845 H atom, probably all of 

 electrical origin due to inertia of self-induction. 



Theory shows that when speed of electron = i/io velocity of light its mass should be appreciably dependent upon 

 that speed. If mo be mass for small velocity v, m be the transverse mass for v, v/( velocity of light) = /8, then m - 

 mo(i /S 2 )^, Lorentz, Einstein; 



for/3 =0.01 o.io 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 



ml mo = 1.00005 1.005 1-02 1.048 1.091 1.155 1.250 1.400 1.667 2.2Q4 



(Confirmed by Bucherer, Ann. d. Phys. 1909, Wolz, Ann. d. Phys. Radium ejects electrons with 3/10 to 98/100 velocity 

 of light.) m, due to charge = 2E 2 /^a, E = charge, a = radius, whence radius of electron = 2 X io~ 13 cm = 1/50,000 

 atomic radius. Cf. (radius of earth)/ (radius of Neptune's orbit) = 1/360,000. 



Positive electron: heavy, extraordinarily small, never found associated with mass less than that of H atom. If 

 mass all electrical (?) radius must be 1/2000 that of the electron. No experimental evidence as with electron, 

 since high enough speeds not available. Penetrability of atom by /3 particle (may penetrate 10,000 atomic systems 

 before it happens to detach an electron) and a particles (8000 times more massive than electron, pass through 500,000 

 atoms without apparent deflection by nucleus more than 2 or 3 times) shows extreme minuteness. Upper limit not 

 larger than icr" cm for Au (heavy atom) or 10'", H (light atom) (Rutherford). Cf. (radius sun)/(radms Neptune's 

 orbit) = 1/3000, but sun is larger than planets. (Hg atoms by billions may pass through thin-walled highly-evacuated 

 glass tubes without impairing vacuum, therefore massive parts of atoms must be extremely small compared to volume 

 of atom.) 



Rutherford atom: number of free + charges on atomic nuclei of different elements = approximately \ atomic 

 weight (Rutherford, Phil. Mag. 21, 1911, deflection of a particles); Barkla concluded free electrons outside 

 nucleus same in number (Phil. Mag. 21, 191 1, X-ray scattering). If mass is electromagnetic, then lack of exact equiva- 

 lence may be due to overlapping fields in heavy crowded atoms, a sort of packing effect; the charge on U = 92, at. wt. 

 = 238.5. Moseley (Phil. Mag. 26, 1912; 27, 1914) photographed and analyzed X-ray spectra, showing their exact 

 similarity in structure from element to element, differing only in frequencies, the square roots of these frequencies 

 forming an arithmetical progression from element to element. Moseley's series of increasing X-ray frequencies is with 

 one or two exceptions that of increasing atomic weights, and these exceptions are less anomalous for the X-ray series 

 than for the atomic -weight series. It seems plausible then that there are 92 elements (from H to U) built up by the 

 addition of some electrical element. Moseley assigned successive integers to this series (see Table 531) known now as 

 atomic numbers. 



Moseley's discovery may be expressed in the form 



i _ Ei A 2 i 

 n 2 " , r Xi ~ Et* 



where E is the nuclear charge and A the wave-length. Substituting for the highest frequency line of W, \t = 0.167 

 X io~ 8 cm (Hull), 2 = 74 = Nw, and Ei = i, then Xi = highest possible frequency by element which has one 

 + electron; Ai = 91.4 wtju. Now the H ultra-violet series highest frequency line = 91.2 my. (Lyman); i.e.. this ultra- 

 violet line of H is nothing but its K X-ray line. Similarly, it seems equally certain that the ordinary Balmer series of 

 H (head at 365 mfj.) is its L X-ray series and Paschen's infra-red series its M X-ray series. 



There may be other electrons on the nucleus (with corresponding -f- charges) since they seem to be shot out 

 by radioactive processes. They may serve to hold the + charges together. He, atomic no. = 2, has 2 free -f- charges, 

 at. wt. = 4; may imagine nucleus has 4 + electrons held together by 2 electrons, with 2 electrons outside nucleus. 

 H has one + and one electron. 



The application of Newton's law to Moseley's law leads to Ei/Ez = ai/a\-, where the a's are the radii of the inmost 

 electronic orbits, i.e., the radii of these orbits are inversely proportional to the central charges or atomic numbers. 



(Note: When an a particle (-f- charge = 2e) is emitted by a radioactive element, its atomic number decreases by 

 2, the emission of a charged particle increases its atomic number by i.) 



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. \Vhen 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. Ii 

 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 = N(i/m" i/n 2 ) where v is the frequency, N, a constant, 

 and i for all the lines in the visible spectrum has the value 2, , the successive integers, 3, 4. 5, . . .; if m = i and , 



2,3,4,. ..Lyman's ultra-violet series results; if m =3,n. 4,5,6 Paschen's infra-red series. These con 



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



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



SMITHSONIAN TABLES. 



(This Table supplements Table 514). 



3 Li 3.00 13 Al 2.70 25 Mn a. 95 t 3& Kr 2.35* 54 Xe 2.70* 



Gl 3 .o .4 Si .35 26 Fe 2.80 37 Rb 4-5 55 4-75 



4 Gl 2.30 14 Si 



6 C 1.54 16 S 



7 N 1.30 .7 Cl 



. 



8 O .30 18 A 



9 F .35 19 



' 



20 r e 2.00 J/ rww ^O" f/J 



27 Co 2.75 38 Sr 3.90 56 Ba 4.20 



28 Ni 2.70 47 Ag 3-55 ' Tl 4.50 



.10 28 INI 2.70 47 "K -*-55 



.05* 29 Cu 2.75 48 Cd 3.20 82 Pb 3-80 



9 r ,.35 .v- .15 30 Zn 2.65 



10 Ne 1.30* ' 20 Ca 3.40 33 As 2.52 



11 Na 3.55 22 Ti 2.80 34 Se 2.3$ 52 2.65 



12 Mg 2.85 24 Cr 2 .8ot 35 Br 2.38 53 I 2.80 



* Outer electron shell. t Cr, "electronegative," 2.35; Mn, ditto, 2.35. 



Broughall (Phil. Mag. 41, p. 872, t 9 2i) computes in the same units from Van der Waal's constant " b " the diame- 

 ters of He, N A, Kr, and^C as 2 7 3 , 2.6, 2.9, 3.1, and 3.4- These inert elements correspond to JW^g2 

 filled successive electron shells. The corresponding atomic numbers are 2, 10, 18, 36 and 54. for Langmu 



filled successive electron shells. The corresponding at 

 see J. Am. Ch. Soc., p. 868, 1919, Science 54, p. 59, i9 21 



