ELECTRICAL STRUCTURE OF MATTER RUTHERFORD 173 



other, and where the orbits described are much more intricate than 

 the orbit of the single electron in hydrogen. Notwithstanding the 

 great difficulties of such a complicated system of electrons in motion, 

 it has been possible to fix the quantum numbers that characterize the 

 motion of each electron, and to form at any rate a rough idea of the 

 character of the orbit. 



These planetary electrons divide themselves up into groups, accord- 

 ing as their orbits are characterized by one or more equal quantum 

 numbers. Without going into detail a few examples may be given 

 to illustrate the conclusions which have been reached. As we have 

 seen, the first element hydrogen has a nuclear charge of 1 and 1 

 electron ; the second, helium, has a charge 2 and 2 electrons, moving 

 in coupled orbits on the detailed nature of which there is still some 

 uncertainty. These two electrons form a definite group, loiown as 

 the K group, which is common to all the elements except hydrogen. 

 For increasing nuclear charge the K group of electrons retain their 

 characteristics, but move with increasing speed, and approach closer 

 to the nucleus. As we pass from helium of atomic number 2 to 

 neon, number 10, a new group of electrons is added consisting of 

 two subgroups, each of four electrons, together called the L group. 

 This L group appears in all atoms of higher atomic number, and, 

 as in the case of the K group, the speed of motion of the electrons 

 increases, and the size of their orbits diminishes with the atomic 

 number. When once the L group has been completed a new and still 

 more complicated M group of electrons begins forming outside it, 

 and a similar process goes on until uranium, which has the highest 

 atomic number, is reached. 



It may be of interest to try to visualize the conception of the 

 atom we have so far reached by taking for illustration the heaviest 

 atom, uranium. At the center of the atom is a minute nucleus sur- 

 rounded by a swirling group of 92 electrons, all in motion in definite 

 orbits and occupying but by no means filling a volume very large 

 compared with that of the nucleus. Some of the electrons describe 

 nearly circular orbits round the nucleus; others, orbits of a more 

 elliptical shape whose axes rotate rapidly round the nucleus. The 

 motion of the electrons in the different groups is not necessarily 

 confined to a definite region of the atom, but the electrons of one 

 group may penetrate deeply into the region mainly occupied by 

 another group, thus giving a type of interconnection or coupling 

 between the various groups. The maximum speed of any electron 

 depends on the closeness of the approach to the nucleus, but the 

 outermost electron will have a minimum speed of more than 1,000 

 kilometers per second, while the innermost K electrons have an 

 average speed of more than 150,000 kilometers per second, or half the 



