DISCOVERY 



43 



what is inside the atom, \-ery much stronger measures 

 must be taken, and these the discovery of X-rays and 

 radio-activity placed in the hands of the phj'sicist. 



\Mien a beam of X-rays traverses matter, a certain 

 amount is scattered in much the same way that the air 

 scatters sunhght and causes the sky to be blue. The 

 amount scattered can be used to get an estimate of the 

 number of scattering centres, the electrons, in much the 

 same way that the brightness of the sky shows how the 

 upper air is constituted. The first estimate of the 

 number of electrons in an atom was derived in this way, 

 and the result came out, though only rather roughly, to 

 be about the same as the value of the atomic weight 

 itself ; at any rate, it could not be supposed that anj' 

 appreciable part of the weight of an atom was con- 

 tributed by its electrons. 



Turning to radio-activity, the evidence becomes far 

 more definite. Radio-activity consists in a succession 

 of transformations of a substance through a perfectly 

 definite series of states. Each state has all the pro- 

 perties associated by the chemist with the word 

 " element," and it is only in the fact of their transmu- 

 tability that the radio-active elements differ from the 

 others. In the act of transformation each atom throws 

 off a high-speed particle. The particle is characteristic 

 of the element that is changing, and there are two 

 different kinds, known as a- and ^-particles. The 

 a-particle is simply a helium atom carrying a positive 

 electric charge equal to twice that of an electron. Its 

 speed depends on the radio-element undergoing trans- 

 formation, and in the case of the element known as 

 Radium C has the enormous value of one-fifteenth of 

 the velocity of light. The /3-particle, on the other hand, 

 is an electron, and its velocity is sometimes almost 

 equal to the velocity of light itself. The speed of both 

 types is so great that they can disregard entirely 

 the impenetrable character of the atom, and go right 

 through it. In doing so they will sometimes go verj' 

 close indeed to one of the electrons in the atom, and 

 will be deflected from their course by it. The /3- 

 particle wiU be easily deflected on account of its light- 

 ness, and indeed it is observed that, if a parallel beam of 

 /3-rays falls on a thin sheet of a substance, the emerg- 

 ing beam is scattered so that it is spread over a broad 

 angle. Not so the a-particles. These are so massive, 

 compared to the electrons, that they go right through 

 everything practically in a straight line, and it requires 

 the cumulative effect of a large number of electrons to 

 produce a perceptible spreading of the beam. Experi- 

 ments were therefore carried out in Rutherford's labora- 

 tory' in Manchester to estimate the amount of this 

 scattering. The results showed that for the substances 

 examined the number of electrons in an atom was 

 equal to about half its atomic weight. 



In the course of the scattering experiments it was 



observed that a few of the a-particles underwent large 

 deflections, much as do /8-particles, and this observa- 

 tion furnished the key to the final solution of the 

 problem. For an a-particle is so heavy that it requires 

 the development of enormous forces to throw it off 

 its line of motion, and these forces can only be caused 

 by another body of large mass and charge. On these 

 grounds Rutherford put forward his theory of atomic 

 structure, which is now universally accepted. An 

 atom has a nucleus of very small dimensions which 

 carries practically the whole of its mass, and has a 

 charge of positive electricity equal to some multiple 

 of the charge of an electron. This charge is neutra- 

 lised b\' a cloud of electrons surrounding it, and the 

 number in this cloud, or (which is the same thing) the 

 number of elementary charges in the nucleus, is the 

 fundamental character which determines the chemical 

 nature of the atom. Radio-active transformation is 

 due to a change of the nucleus, and as some transforma- 

 tions give /3-particles (which are electrons), the nucleus 

 certainly contains electrons in itself; but these are 

 merged in its total charge, and are not to be counted 

 in estimating the number of electrons in the atom. 

 Chemical processes are only strong enough to scrape 

 off an electron or two, or to make some of the outer 

 ones vibrate, and it requires X-rays or a- or /S-particles 

 to get inside and show the qualities of the nucleus and 

 the inner electrons. The laws of the forces ruling in 

 the atom, and still more in the nucleus, are almost 

 completely unknown at present, and constitute one of 

 the most important and difficult problems of physics. 



To test Rutherford's theory, very intense radio-active 

 sources were prepared so that considerable numbers of 

 a-particles would be deflected to broad angles, and these 

 numbers were counted. The results bore out the 

 theory completelj', and indicated nuclei of strength 

 equal to a half or a little less than half of the atomic 

 weight. This was found for gold, platinum, tin, silver, 

 copper, aluminium, and carbon. An eighth case can be 

 at once added, the a-particle itself. In the course of 

 its violent history it is certain that any electron 

 attached to it would be scraped off. In other words, 

 it is itself the nucleus of the helium atom, and its charge, 

 2, is half its atomic weight, 4. 



Now if the elements are numbered off, starting with 

 hydrogen as i, and going in order of their atomic 

 weights, it will be observed that the resulting numbers 

 are all (excepting h3'drogen itself) half the atomic 

 weight or a little less. So there is a very strong sug- 

 gestion that it is this number that is really being given 

 by the experiments. If the a-particle experiment 

 were susceptible of a high degree of accuracy', we should 

 have a perfectly direct way of counting what the exact 

 number is in each case, and could in this way also 

 discover any gaps in the series. But to get even rough 



