492 Table 603 (a) 



ELECTRONS, PROTONS, ATOMIC STRUCTURE 



Free negative electron (corpuscle, J. J. Thomson). — Mass, spectroscopic (bound) 

 9-035 X io" 28 g; free, 8.994 X io" 28 g; atomic weight, 5.479 and 5.454 X io" 1 respectively, 

 probably all electrical, due to inertia of self-induction. Theory shows that when speed of 

 electron = 1/10 velocity of light, its mass should be appreciably dependent upon that speed. 

 If mo be the mass for small velocity, m, the transverse mass for v, and r/( velocity of light, 

 c) =(i then m= m« (1 — j3 2 )"s (Lorentz, Einstein). 



/3, 0.01 0.10 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 



m/mo, 1.00005 1.005 i-O 2 1.048 1.091 1. 155 1.250 1.400 1.667 2.294 



Radium ejects electrons with 3/10 to 98/100 of c. m, due to charge = 2E 2 /2,a, where 

 E = charge, a, radius, whence radius of electron is 2 X io~ 13 cm = 1/50,000 atomic radius. 

 Cf. (radius earth/radius Neptune's orbit) = 1/360,000. Collisions with a particles show 

 diameter of electron must be less than 4 X io~ 13 cm (Chadwick, Bieler, Philos. Mag., 1921). 



Positive electron or proton. — Heavy, extraordinarily small, never found associated with 

 mass less than that of the H atom; mass, 1.6609 X io -24 g. Specific charge, 9579.7 abs-em- 

 units ■ g"\ Ratio mass proton to mass electron, 1838 (spectroscopic), 1847 (deflection). 

 If mass is all electrical, radius must be 1/2000 that of electron. No experimental evidence 

 as with the latter since high enough speeds not available. Penetrability of atom by 

 /? particle (may penetrate 10,000 atomic systems before it happens to detach an electron) 

 and a particle (8000 times more massive than negative electron, passes through 500,000 

 atoms without apparent deflection by nucleus more than 2 or 3 times) shows extreme 

 minuteness. Upper limit of nucleus not larger than io" 12 cm for Au (heavy atom) and 

 IO" 13 cm for H (light atom) (Rutherford). Cf. (radius sun)/(radius Neptune's orbit) 

 = 1/3000 but sun larger than planets. Hg atoms by billions may pass through thin-walled, 

 highly-evacuated glass tube without impairing vacuum, therefore massive parts of atoms 

 must be extremely small compared to volume of atom. 



Rutherford atom. — Atoms of all elements are somewhat similarly built. At the center 

 a charged nucleus of minute dimensions, responsible for most of the mass of the atom ; this 

 is surrounded by a distribution held in equilibrium by the force from the nucleus. Resultant 

 nuclear charge = atomic or ordinal no., varies from 1 for H to 92 for U. These atomic 

 nos. represent the number of planetary electrons which surround the nucleus. By the 

 action of light, the electric charge, bombardment by a particles, one or more of the 

 planetary electrons may be driven away from the nucleus ; by X rays or the swift )S rays 

 some of the more strongly bound may be removed. New electrons are generally soon 

 captured to replace these. The nucleus is much more stable and when disrupted (radio- 

 active changes, bombardment with a particles) shows no tendency to revert to original 

 state. 



Moseley (Philos. 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 the frequencies forming an arithmetical progression from 

 element to element. Moseley's series of increasing X-ray frequencies is with one or two 



Smithsonian Tables 



