IIUO] on X-Rays and Atomic Structure Till 



At about the same time as the aiinouiicement of these corrected 

 vahies, Rutherford from experiments on the scattering of a particles 

 arrived at a concUision which is complementary to this. He con- 

 cluded that the deflections of a particles observed are not such as 

 could be explained either by the electrons or the diffuse positive 

 charge in the atom. He showed it was necessary to replace the 

 diffuse positive charge by a very concentrated positive charge, and 

 found for it a value approximately equal to half the atomic weight 



times the charge of an electron, i.e. ~ e. This just agrees— though 



the measurements did not permit of equal accuracy — with the 



electrons with charge - e. The two are complementary and give 



us a neutral atom. 



Later work by Moseley on the spectra of the elements given by 

 interference phenomena with X-rays has further supported this 

 conclusion, by showing equal steps between the spectrum of each 

 atom and that of the next in the periodic series, each step roughly 

 corresponding to an increase in the atomic weight by two units. 



Bohr has extended the theory to account for the line spectra of 

 light elements. 



This, then, is the structure which at present perhaps finds greatest 

 acceptance — a very small nuclear positive charge of unknown but 

 probably quite complicated structure surrounded by a number of 

 electrons at various distances from the centre ; the positive nucleus 

 having a charge approximately equal to half the atomic weight times 

 the charge of an electron (e), now more accurately expressed as the 

 atomic number {ii) times (e) ; the number of negative electrons outside 

 the nucleus being approximately half the atomic weight, or, more 

 accurately, equal to the atomic number. 



It is evident that a great burden is placed on the solitary electron 

 which with the positive nucleus constitutes a hydrogen atom. To 

 get over the difficulty Bohr has suggested that the electron may be 

 in one of many distinct orbits. The normal hydrogen atom is 

 supposed to have a diameter 1 • 1 x 10"^ cm., but according to Bohr's 

 theory it also may be 4, 9, 16, 25 times this, and to explain stellar 

 spectra must be 1*2 x 10~^ cm. in diameter. 



Writing of this theory Jeans says " it explains so many facts, and 

 the numerical agreement with observation is so complete, that little 

 doubt will be felt that it is at least based on a very substantial 

 substratum of truth." 



Before proceeding to explain how we can get further information 

 of the atoms from X-ray experiments, let us look at the theory of 

 scattered radiation which enabled us to calculate the number of 

 electrons. It is briefly this : The primary waves spread in all 

 directions from the anti-kathode of an X-ray tube and influence every 

 electron in the matter through which they pass. AVhatever the 



