THE PRESIDENTIAL ADDRESS. 13 
as their orbits are characterised 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, known 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 sub-groups, 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 visualise the conception of the atom 
we have so far reached by taking for illustration the heaviest atom, 
uranium. At the centre of the atom is a minute nucleus surrounded 
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 
inter-connection or coupling between the various groups. The maxi- 
mum 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 kilometres per second, while the innermost K elec- 
trons have an average speed of more than 150,000 kilometres per 
second, or half the speed of light. When we visualise the extraordinary 
complexity of the electronic system we may be surprised that it has 
been possible to find any order in the apparent medley of motions. 
In reaching these conclusions, which we owe largely to Professor 
Bohr and his co-workers, every available kind of data about the different 
atoms has been taken into consideration. A study of the X-ray spectra, 
in particular, affords information of great value as to the arrangement 
of the various groups in the atom; while the optical spectrum and 
general chemical properties are of great importance in deciding the 
