32 SECTIONAL ADDRESSES 



the primitive concept of the nuclear atom, with its nucleus composed of 

 ( x -j_ #) protons and * electrons, so that the nuclear charge was ze (e being 

 the electronic or protonic charge), and electrical neutrality was assured 

 by assuming that z satellite-electrons (z being what is called the atomic 

 number) circulated in orbits around the nucleus. 



The inevitable consequences of the existence of such atoms radiating 

 according to classical laws, was an unstable universe in which the satellite- 

 electrons, radiating energy as they revolve, would spiral down towards 

 the nucleus and finally collapse therein. Quantum notions saved the 

 concept, and one of the peaks in the development of twentieth- century 

 physics is the story of the Bohr atom, in which it is assumed that only a 

 restricted number of stable orbits, or states, is possible ; that electrons 

 in these orbits do not radiate ; that an electron in moving from one orbit 

 to another radiates or absorbs quanta of energy equal to the difference 

 between the energy states of the two orbits, and that the angular momen- 

 tum is quantised, that is, is restricted to a number of discrete values, the 

 magnitude of the value in the JVth orbit being Nh /zn. 



The application of a little simple algebra to the expression of these 

 postulates results in an equation which represents the disposition of the 

 lines in the spectrum of a single- electron atom, such as hydrogen, or ionised 

 helium, with considerable accuracy. The theory is easily extended to 

 elliptic orbits, though here, having to deal with a varying radius vectorand 

 varying radial momentum, we have to quantise this latter quantity and two 

 quantum numbers become necessary, the so-called azimuthal quantum 

 number (k) which quantises the angular momentum, and what is called 

 the radial quantum number, the sum of the two being set equal to the 

 total quantum number (N). 



But the theory in this form was quite inadequate to cope with any 

 system more complex than a single electron system. To deal with these 

 more complex systems , quantum notions were extended on quasi- 

 empirical lines and resulted in what may be called a vector model of the 

 atom in which were visualised the possibility of electron and nuclear 

 spins, with further possibilities in the way of quantisation and quantum 

 numbers. If these quantum numbers are shared between the satellite- 

 electrons of an atom in such a way as to agree with an empirical exclusion 

 principle which states that no two electrons in an atom may have all their 

 quantum numbers identical, we may arrive at a distribution of the satellite- 

 electrons as regards their energy-levels which gives a model capable of 

 explaining many complex spectroscopic (and other) facts. 



But space presses and we must return, in this rapid survey, to a con- 

 sideration of that dualism of outlook which appeared so early in the story 

 of twentieth-century physics. The discovery of the Compton effect 

 further emphasised this corpuscular aspect of radiation. 2 



2 When X-rays are scattered by impact with the more lightly bound electrons 

 in an atom, the radiation scattered at an acute angle has a smaller frequency 

 than the frequency of the incident radiation, a simple explanation of the 

 change being at once forthcoming if the problem is treated in the manner of 

 the treatment of the impact of elastic spheres. Thus a light quantum hn 

 communicates kinetic energy to an electron by impact. The scattered 

 quantum hn' will have less energy, and hence ri will be less than n. 



